diff --git a/AGENTS.md b/AGENTS.md
index 168db54..7ab89a3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -84,12 +84,14 @@ cd frontend
 npm run dev
 npm run build
 npm run lint
+npm run test
+npm run test:run
 npm run preview
 ```
 
 `npm run build` runs `tsc -b && vite build`, then `postbuild` runs `node scripts/generate-sitemap.js`.
 
-Do not write or report `npm test` as a repository command unless a `test` script is added to `frontend/package.json`.
+`npm run test` runs Vitest in watch mode. Use `npm run test:run` for a non-watch frontend test run.
 
 ## Coding style
 
@@ -122,13 +124,49 @@ Do not write or report `npm test` as a repository command unless a `test` script
 - Keep page-level code under `frontend/src/pages` and shared UI/feature code under `frontend/src/components` unless existing patterns point elsewhere.
 - Match existing Tailwind, component, and import alias conventions.
 
+### Frontend TSX style
+
+- Preserve the local TSX formatting style. Do not normalize TSX to common Prettier-style React formatting unless explicitly asked.
+- Prefer `const` arrow functions for TypeScript/TSX component and helper declarations.
+- Put two blank lines before and after top-level `const` function declarations, unless imports, exports, or file boundaries make that awkward.
+- In TSX, indent nested tag attributes with one tab relative to the tag line. With the project tab width, this visually appears as 4 spaces.
+- Keep a tag's closing marker on the same line as the final prop when the tag spans multiple lines. Do not put `/>` or `>` on its own line unless the existing surrounding code does so.
+- Keep JSX closing parentheses in the existing compact style, for example `</div>)` rather than moving `)` onto a separate line.
+
+Preferred:
+
+```tsx
+const PostFormTagsArea: FC<Props> = ({ tags, setTags, errors, ...rest }) => {
+  return (
+      <TextArea
+	  {...rest}
+	  ref={ref}
+	  value={tags}
+	  invalid={errors && errors.length > 0}
+	  onChange={ev => setTags (ev.target.value)}/>)
+}
+```
+
+Avoid:
+
+```tsx
+function PostFormTagsArea ({ tags, setTags }: Props) {
+  return (
+    <TextArea
+      value={tags}
+      onChange={ev => setTags (ev.target.value)}
+    />
+  )
+}
+```
+
 ## Codex workflow
 
 - First inspect existing patterns; do not invent new architecture when a local convention exists.
 - Keep changes scoped to the requested issue.
 - Do not scan or summarize dependency/generated/runtime directories such as `node_modules`, `dist`, `tmp`, `log`, and `storage` unless explicitly needed.
 - Before touching wiki, tag, versioning, BAN, IP BAN, or authentication behavior, inspect the related request specs and service objects.
-- If frontend code changes, run the existing frontend verification commands that apply: `npm run build` and `npm run lint`.
+- If frontend code changes, run the existing frontend verification commands that apply: `npm run build`, `npm run lint`, and `npm run test:run`.
 - If backend code changes, run the relevant RSpec command; for broad backend changes, run `bundle exec rspec`.
 - If a verification command cannot be run or fails, report the exact command and failure.
 
diff --git a/backend/Gemfile b/backend/Gemfile
index 2d0a90c..7b44641 100644
--- a/backend/Gemfile
+++ b/backend/Gemfile
@@ -69,3 +69,5 @@ gem 'discard'
 gem "rspec-rails", "~> 8.0", :groups => [:development, :test]
 
 gem 'aws-sdk-s3', require: false
+
+gem 'rails-i18n', '~> 8.0.0'
diff --git a/backend/Gemfile.lock b/backend/Gemfile.lock
index f9dc02c..555f867 100644
--- a/backend/Gemfile.lock
+++ b/backend/Gemfile.lock
@@ -306,6 +306,9 @@ GEM
     rails-html-sanitizer (1.6.2)
       loofah (~> 2.21)
       nokogiri (>= 1.15.7, != 1.16.7, != 1.16.6, != 1.16.5, != 1.16.4, != 1.16.3, != 1.16.2, != 1.16.1, != 1.16.0.rc1, != 1.16.0)
+    rails-i18n (8.0.2)
+      i18n (>= 0.7, < 2)
+      railties (>= 8.0.0, < 9)
     railties (8.0.2)
       actionpack (= 8.0.2)
       activesupport (= 8.0.2)
@@ -477,6 +480,7 @@ DEPENDENCIES
   puma (>= 5.0)
   rack-cors
   rails (~> 8.0.2)
+  rails-i18n (~> 8.0.0)
   rspec-rails (~> 8.0)
   rubocop-rails-omakase
   sprockets-rails
diff --git a/backend/app/controllers/application_controller.rb b/backend/app/controllers/application_controller.rb
index 4f3d6ed..42adc75 100644
--- a/backend/app/controllers/application_controller.rb
+++ b/backend/app/controllers/application_controller.rb
@@ -1,4 +1,7 @@
 class ApplicationController < ActionController::API
+  rescue_from ActiveRecord::RecordInvalid, with: :render_record_invalid
+  rescue_from ActiveRecord::RecordNotUnique, with: :render_record_not_unique
+
   before_action :reject_banned_ip_address!
   before_action :authenticate_user
   before_action :reject_banned_user!
@@ -25,6 +28,27 @@ class ApplicationController < ActionController::API
     end
   end
 
+  def render_bad_request message = 'リクエストが不正です．'
+    render json: { type: 'bad_request',
+                   message:,
+                   errors: { },
+                   base_errors: [message] },
+           status: :bad_request
+  end
+
+  def render_unprocessable_entity message = '入力を確認してください．', field: nil
+    render_validation_error(fields: field ? { field => [message] } : { },
+                            base: field ? [] : [message])
+  end
+
+  def render_record_invalid error
+    render_validation_error error.record
+  end
+
+  def render_record_not_unique _error = nil
+    render_validation_error base: ['すでに存在してゐます．']
+  end
+
   def reject_banned_ip_address!
     ip_address = IpAddress.find_by(ip_address: IPAddr.new(request.remote_ip).hton)
     return unless ip_address&.banned?
@@ -37,4 +61,28 @@ class ApplicationController < ActionController::API
 
     head :forbidden
   end
+
+  def render_validation_error record = nil, fields: { }, base: [], status: :unprocessable_entity
+    errors = { }
+
+    if record
+      record.errors.each do |error|
+        errors[error.attribute] ||= []
+        errors[error.attribute] << error.message
+      end
+    end
+
+    fields.each do |attr, messages|
+      errors[attr.to_sym] ||= []
+      errors[attr.to_sym].concat(Array(messages))
+    end
+
+    base_errors = Array(base) + Array(errors.delete(:base))
+
+    render json: { type: 'validation_error',
+                   message: '入力内容を確認してください．',
+                   errors:,
+                   base_errors: },
+           status:
+  end
 end
diff --git a/backend/app/controllers/deerjikists_controller.rb b/backend/app/controllers/deerjikists_controller.rb
index dff9d82..9302370 100644
--- a/backend/app/controllers/deerjikists_controller.rb
+++ b/backend/app/controllers/deerjikists_controller.rb
@@ -2,7 +2,8 @@ class DeerjikistsController < ApplicationController
   def show
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
-    return head :bad_request if platform.blank? || code.blank?
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
 
     deerjikist = Deerjikist
                  .joins(:tag)
@@ -22,7 +23,9 @@ class DeerjikistsController < ApplicationController
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
     tag_id = params[:tag_id].to_i
-    return head :bad_request if platform.blank? || code.blank? || tag_id <= 0
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
+    return render_bad_request('tag_id が不正です．') if tag_id <= 0
 
     deerjikist = Deerjikist.find_or_initialize_by(platform:, code:).tap do |d|
       d.tag_id = tag_id
@@ -38,7 +41,8 @@ class DeerjikistsController < ApplicationController
 
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
-    return head :bad_request if platform.blank? || code.blank?
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
 
     Deerjikist.find([platform, code]).destroy!
 
diff --git a/backend/app/controllers/materials_controller.rb b/backend/app/controllers/materials_controller.rb
index d61a4b3..2a86b77 100644
--- a/backend/app/controllers/materials_controller.rb
+++ b/backend/app/controllers/materials_controller.rb
@@ -40,7 +40,11 @@ class MaterialsController < ApplicationController
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     url = params[:url].to_s.strip.presence
-    return head :bad_request if tag_name_raw.blank? || (file.blank? && url.blank?)
+    return render_unprocessable_entity('タグは必須です．', field: :tag) if tag_name_raw.blank?
+    if file.blank? && url.blank?
+      return render_validation_error fields: { file: ['ファイルまたは URL は必須です．'],
+                                               url: ['ファイルまたは URL は必須です．'] }
+    end
 
     tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
     tag = tag_name.tag
@@ -54,7 +58,7 @@ class MaterialsController < ApplicationController
     if material.save
       render json: MaterialRepr.base(material, host: request.base_url), status: :created
     else
-      render json: { errors: material.errors.full_messages }, status: :unprocessable_entity
+      render_validation_error material
     end
   end
 
@@ -68,7 +72,11 @@ class MaterialsController < ApplicationController
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     url = params[:url].to_s.strip.presence
-    return head :bad_request if tag_name_raw.blank? || (file.blank? && url.blank?)
+    return render_unprocessable_entity('タグは必須です．', field: :tag) if tag_name_raw.blank?
+    if file.blank? && url.blank?
+      return render_validation_error fields: { file: ['ファイルまたは URL は必須です．'],
+                                               url: ['ファイルまたは URL は必須です．'] }
+    end
 
     tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
     tag = tag_name.tag
@@ -84,7 +92,7 @@ class MaterialsController < ApplicationController
     if material.save
       render json: MaterialRepr.base(material, host: request.base_url)
     else
-      render json: { errors: material.errors.full_messages }, status: :unprocessable_entity
+      render_validation_error material
     end
   end
 
diff --git a/backend/app/controllers/nico_tags_controller.rb b/backend/app/controllers/nico_tags_controller.rb
index 087c592..ce63840 100644
--- a/backend/app/controllers/nico_tags_controller.rb
+++ b/backend/app/controllers/nico_tags_controller.rb
@@ -1,26 +1,69 @@
 class NicoTagsController < ApplicationController
   def index
-    limit = (params[:limit] || 20).to_i
-    cursor = params[:cursor].presence
+    name = params[:name].presence
+    linked_tag = params[:linked_tag].presence
+    link_status = params[:link_status].presence
+    order = params[:order].to_s.split(':', 2).map(&:strip)
+    order[0] = 'updated_at' unless order[0].in?(['name', 'created_at', 'updated_at'])
+    unless order[1].in?(['asc', 'desc'])
+      order[1] = order[0] == 'name' ? 'asc' : 'desc'
+    end
+    page = (params[:page].presence || 1).to_i
+    limit = (params[:limit].presence || 20).to_i
+
+    page = 1 if page < 1
+    limit = 1 if limit < 1
+
+    post_tag_max_sql =
+        PostTag
+        .select('tag_id, MAX(created_at) AS max_created_at')
+        .group('tag_id')
+        .to_sql
 
     q = Tag.nico_tags
+           .joins(:tag_name)
+           .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
+                  'ON post_tag_max.tag_id = tags.id')
            .includes(:tag_name, tag_name: :wiki_page, linked_tags: { tag_name: :wiki_page })
-           .order(updated_at: :desc)
-    q = q.where('tags.updated_at < ?', Time.iso8601(cursor)) if cursor
-
-    tags = q.limit(limit + 1).to_a
-
-    next_cursor = nil
-    if tags.size > limit
-      next_cursor = tags.last.updated_at.iso8601(6)
-      tags = tags.first(limit)
+    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
+    if linked_tag
+      linked_tag_ids =
+        Tag
+        .joins(:tag_name)
+        .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
+        .pluck(:id)
+      linked_nico_tag_ids = NicoTagRelation.where(tag_id: linked_tag_ids).pluck(:nico_tag_id)
+      q = q.where(id: linked_nico_tag_ids)
+    end
+    if link_status.in?(['linked', 'unlinked'])
+      exists_sql =
+          'EXISTS (SELECT 1 FROM nico_tag_relations ' \
+          'WHERE nico_tag_relations.nico_tag_id = tags.id)'
+      q = link_status == 'linked' ? q.where(exists_sql) : q.where("NOT #{ exists_sql }")
     end
 
+    count = q.count
+    sort_sql =
+        case order[0]
+        when 'name'
+          'tag_names.name'
+        when 'updated_at'
+          'post_tag_max.max_created_at'
+        else
+          "tags.#{ order[0] }"
+        end
+    tags = q.reselect('tags.*',
+                      Arel.sql('post_tag_max.max_created_at AS recent_post_tag_created_at'))
+            .order(Arel.sql("#{ sort_sql } #{ order[1] }, tags.id #{ order[1] }"))
+            .limit(limit)
+            .offset((page - 1) * limit)
+            .to_a
+
     render json: { tags: tags.map { |tag|
-      TagRepr.base(tag).merge(linked_tags: tag.linked_tags.map { |lt|
-        TagRepr.base(lt)
-      })
-    }, next_cursor: }
+      TagRepr.base(tag).merge(
+          recent_post_tag_created_at: tag.recent_post_tag_created_at,
+          linked_tags: tag.linked_tags.map { |lt| TagRepr.base(lt) })
+    }, count: }
   end
 
   def update
@@ -30,14 +73,18 @@ class NicoTagsController < ApplicationController
     id = params[:id].to_i
 
     tag = Tag.find(id)
-    return head :bad_request unless tag.nico?
+    return render_bad_request('ニコニコ・タグを指定してください．') unless tag.nico?
 
     linked_tag_names = params[:tags].to_s.split
-    linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
-                                                        with_no_deerjikist: false)
-    return head :bad_request if linked_tags.any? { |t| t.nico? }
+    linked_tags = nil
 
     ApplicationRecord.transaction do
+      linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
+                                                          with_no_deerjikist: false)
+      if linked_tags.any? { |t| t.nico? }
+        raise Tag::NicoTagNormalisationError
+      end
+
       TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 
       tag.linked_tags = linked_tags
@@ -47,5 +94,21 @@ class NicoTagsController < ApplicationController
     end
 
     render json: tag.linked_tags.map { |t| TagRepr.base(t) }, status: :ok
+  rescue Tag::NicoTagNormalisationError
+    render_validation_error fields: { tags: ['ニコニコ・タグ同士は連携できません．'] }
+  rescue ActiveRecord::RecordInvalid => e
+    render_nico_tag_form_record_invalid e.record
+  end
+
+  private
+
+  def render_nico_tag_form_record_invalid record
+    if record.is_a?(TagName) || record.is_a?(Tag)
+      render_validation_error fields: { tags: record.errors.full_messages.map { |message|
+        "タグ名 “#{ record.name }”: #{ message }"
+      } }
+    else
+      render_validation_error record
+    end
   end
 end
diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index e8cae00..7bddd07 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -44,7 +44,7 @@ class PostsController < ApplicationController
         filtered_posts
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
-        .preload(tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
+        .preload(:uploaded_user, tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -95,7 +95,9 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
+    post = filtered_posts.preload(:uploaded_user,
+                                  tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
+                         .with_attached_thumbnail
                          .order('RAND()')
                          .first
     return head :not_found unless post
@@ -104,12 +106,24 @@ class PostsController < ApplicationController
   end
 
   def show
-    post = Post.includes(tags: [:deerjikists, :materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
+    post =
+        Post
+        .includes(:uploaded_user, tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
+        .with_attached_thumbnail
+        .find_by(id: params[:id])
     return head :not_found unless post
 
-    render json: PostRepr.base(post, current_user)
-                         .merge(tags: build_tag_tree_for(post.tags),
-                                related: PostRepr.many(post.related(limit: 20)))
+    parent_posts = post.parents.with_attached_thumbnail.order(:id).to_a
+    child_posts = post.children.with_attached_thumbnail.order(:id).to_a
+    sibling_posts = sibling_posts_by_parent(parent_posts.map(&:id))
+    related = post.related(limit: 20).to_a
+
+    render json: PostRepr.detail(post, current_user,
+                                 parent_posts:,
+                                 child_posts:,
+                                 sibling_posts:,
+                                 related:)
+                         .merge(tags: build_tag_tree_for(post.tags))
   end
 
   def create
@@ -148,11 +162,11 @@ class PostsController < ApplicationController
     post.reload
     render json: PostRepr.base(post), status: :created
   rescue Tag::NicoTagNormalisationError
-    head :bad_request
+    render_validation_error fields: { tags: 'ニコニコ・タグは直接指定できません．' }
   rescue ArgumentError => e
-    render json: { errors: [e.message] }, status: :unprocessable_entity
+    render_validation_error fields: { parent_post_ids: [e.message] }
   rescue ActiveRecord::RecordInvalid => e
-    render json: { errors: e.record.errors.full_messages }, status: :unprocessable_entity
+    render_post_form_record_invalid e.record
   end
 
   def viewed
@@ -175,10 +189,10 @@ class PostsController < ApplicationController
 
     force = bool?(:force)
     merge = bool?(:merge)
-    return head :bad_request if force && merge
+    return render_bad_request('force と merge は同時に指定できません．') if force && merge
 
     base_version_no = parse_base_version_no
-    return head :bad_request if !(force) && !(base_version_no)
+    return render_bad_request('base_version_no は必須です．') if !(force) && !(base_version_no)
 
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
@@ -238,11 +252,11 @@ class PostsController < ApplicationController
     json['tags'] = build_tag_tree_for(post.tags)
     render json:, status: :ok
   rescue Tag::NicoTagNormalisationError
-    head :bad_request
+    render_validation_error fields: { tags: ['ニコニコ・タグは直接指定できません．'] }
   rescue ArgumentError => e
-    render json: { errors: [e.message] }, status: :unprocessable_entity
+    render_validation_error fields: { parent_post_ids: [e.message] }
   rescue ActiveRecord::RecordInvalid => e
-    render json: { errors: e.record.errors.full_messages }, status: :unprocessable_entity
+    render_post_form_record_invalid e.record
   end
 
   def changes
@@ -385,7 +399,7 @@ class PostsController < ApplicationController
       return nil unless tag
 
       if path.include?(tag_id)
-        return TagRepr.base(tag).merge(children: [])
+        return TagRepr.inline(tag).merge(children: [])
       end
 
       if memo.key?(tag_id)
@@ -397,12 +411,26 @@ class PostsController < ApplicationController
 
       children = child_ids.filter_map { |cid| build_node.(cid, new_path) }
 
-      memo[tag_id] = TagRepr.base(tag).merge(children:)
+      memo[tag_id] = TagRepr.inline(tag).merge(children:)
     end
 
     root_ids.filter_map { |id| build_node.call(id, []) }
   end
 
+  def sibling_posts_by_parent parent_post_ids
+    return { } if parent_post_ids.blank?
+
+    implications =
+        PostImplication
+        .where(parent_post_id: parent_post_ids)
+        .includes(post: { thumbnail_attachment: :blob })
+        .order(:parent_post_id, :post_id)
+
+    implications.group_by(&:parent_post_id).transform_values { |items|
+      items.map(&:post)
+    }
+  end
+
   def parse_parent_post_ids
     raise ArgumentError, 'parent_post_ids は必須です．' unless params.key?(:parent_post_ids)
 
@@ -416,7 +444,7 @@ class PostsController < ApplicationController
 
   def sync_parent_posts! post, parent_post_ids
     if parent_post_ids.include?(post.id)
-      post.errors.add(:base, '自分自身を親投稿にはできません．')
+      post.errors.add :parent_post_ids, '自分自身を親投稿にはできません．'
       raise ActiveRecord::RecordInvalid, post
     end
 
@@ -424,7 +452,8 @@ class PostsController < ApplicationController
     missing_ids = parent_post_ids - existing_ids
 
     if missing_ids.present?
-      post.errors.add(:base, "存在しない親投稿 ID があります: #{ missing_ids.join(' ') }")
+      post.errors.add :parent_post_ids,
+                      "存在しない親投稿 Id. があります: #{ missing_ids.join(' ') }"
       raise ActiveRecord::RecordInvalid, post
     end
 
@@ -640,4 +669,14 @@ class PostsController < ApplicationController
 
     merged.uniq.sort
   end
+
+  def render_post_form_record_invalid record
+    if record.is_a?(TagName) || record.is_a?(Tag)
+      render_validation_error fields: { tags: record.errors.full_messages.map { |message|
+        "タグ名 “#{ record.name }”: #{ message }"
+      } }
+    else
+      render_validation_error record
+    end
+  end
 end
diff --git a/backend/app/controllers/preview_controller.rb b/backend/app/controllers/preview_controller.rb
index 0590891..5486838 100644
--- a/backend/app/controllers/preview_controller.rb
+++ b/backend/app/controllers/preview_controller.rb
@@ -4,7 +4,7 @@ class PreviewController < ApplicationController
     return head :unauthorized unless current_user
 
     url = params[:url]
-    return head :bad_request unless url.present?
+    return render_bad_request('URL は必須です．') unless url.present?
 
     unless url.start_with?(/http(s)?:\/\//)
       url = 'http://' + url
@@ -16,7 +16,7 @@ class PreviewController < ApplicationController
 
     render json: { title: title }
   rescue => e
-    render json: { error: e.message }, status: :bad_request
+    render_bad_request(e.message)
   end
 
   def thumbnail
@@ -25,7 +25,7 @@ class PreviewController < ApplicationController
     return head :unauthorized unless current_user
 
     url = params[:url]
-    return head :bad_request if url.blank?
+    return render_bad_request('URL は必須です．') if url.blank?
 
     unless url.start_with?(/http(s)?:\/\//)
       url = 'http://' + url
@@ -40,7 +40,11 @@ class PreviewController < ApplicationController
       File.delete(path) rescue nil
       send_file image.path, type: 'image/png', disposition: 'inline'
     else
-      render json: { error: 'Failed to generate thumbnail' }, status: :internal_server_error
+      render json: { type: 'internal_server_error',
+                     message: 'サムネールを生成できませんでした．',
+                     errors: { },
+                     base_errors: ['サムネールを生成できませんでした．'] },
+             status: :internal_server_error
     end
   end
 end
diff --git a/backend/app/controllers/tag_children_controller.rb b/backend/app/controllers/tag_children_controller.rb
index 8eb972e..cfdde30 100644
--- a/backend/app/controllers/tag_children_controller.rb
+++ b/backend/app/controllers/tag_children_controller.rb
@@ -5,11 +5,12 @@ class TagChildrenController < ApplicationController
 
     parent_id = params[:parent_id]
     child_id = params[:child_id]
-    return head :bad_request if parent_id.blank? || child_id.blank?
+    return render_bad_request('parent_id は必須です．') if parent_id.blank?
+    return render_bad_request('child_id は必須です．') if child_id.blank?
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
-    return head :bad_request if parent.nico? || child.nico?
+    return render_bad_request('ニコニコ・タグの階層は変更できません．') if parent.nico? || child.nico?
 
     ApplicationRecord.transaction do
       TagVersioning.ensure_snapshot!(child, created_by_user: current_user)
@@ -27,11 +28,12 @@ class TagChildrenController < ApplicationController
 
     parent_id = params[:parent_id]
     child_id = params[:child_id]
-    return head :bad_request if parent_id.blank? || child_id.blank?
+    return render_bad_request('parent_id は必須です．') if parent_id.blank?
+    return render_bad_request('child_id は必須です．') if child_id.blank?
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
-    return head :bad_request if parent.nico? || child.nico?
+    return render_bad_request('ニコニコ・タグの階層は変更できません．') if parent.nico? || child.nico?
 
     ApplicationRecord.transaction do
       TagVersioning.ensure_snapshot!(child, created_by_user: current_user)
diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index aed029f..30d822b 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -168,7 +168,7 @@ class TagsController < ApplicationController
 
   def show_by_name
     name = params[:name].to_s.strip
-    return head :bad_request if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, :materials, tag_name: :wiki_page)
@@ -192,7 +192,7 @@ class TagsController < ApplicationController
 
   def deerjikists_by_name
     name = params[:name].to_s.strip
-    return head :bad_request if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, tag_name: :wiki_page)
@@ -214,21 +214,24 @@ class TagsController < ApplicationController
 
     ApplicationRecord.transaction do
       tag.deerjikists = []
-      params[:_json].each do
-        platform = _1[:platform]
-        code = normalise_deerjikist_code(platform, _1[:code])
+      params[:_json].each.with_index do |item, i|
+        platform = item[:platform]
+        code = normalise_deerjikist_code(platform, item[:code])
         deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
         deerjikist.tag = tag
-        deerjikist.save!
+        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
+        raise ActiveRecord::Rollback if performed?
       end
     end
 
+    return if performed?
+
     render json: DeerjikistRepr.many(tag.reload.deerjikists)
   end
 
   def materials_by_name
     name = params[:name].to_s.strip
-    return head :bad_request if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, :materials, tag_name: :wiki_page)
@@ -247,17 +250,16 @@ class TagsController < ApplicationController
 
     name = params[:name].to_s.strip
     category = params[:category].to_s.strip
-    return head :unprocessable_entity if name.blank? || category.blank?
+    return render_unprocessable_entity('名前は必須です．', field: :name) if name.blank?
+    return render_unprocessable_entity('カテゴリは必須です．', field: :category) if category.blank?
 
     if name != tag.name &&
         tag.in?([Tag.tagme, Tag.bot, Tag.no_deerjikist, Tag.video, Tag.niconico])
-      return render json: { error: 'システム・タグの名称は変更できません．' },
-                    status: :unprocessable_entity
+      return render_unprocessable_entity('システム・タグの名称は変更できません．', field: :name)
     end
 
     if tag.nico? || category == 'nico'
-      return render json: { error: 'ニコタグは変更できません．' },
-                    status: :unprocessable_entity
+      return render_unprocessable_entity('ニコタグは変更できません．', field: :category)
     end
 
     alias_names = params[:aliases].to_s.split.uniq
@@ -302,8 +304,7 @@ class TagsController < ApplicationController
     tag = Tag.find(params[:id])
 
     if tag.nico? || (category.present? && category == 'nico')
-      return render json: { error: 'ニコタグは変更できません．' },
-                    status: :unprocessable_entity
+      return render_unprocessable_entity('ニコタグは変更できません．', field: :category)
     end
 
     ApplicationRecord.transaction do
@@ -437,4 +438,23 @@ class TagsController < ApplicationController
   rescue
     nil
   end
+
+  def render_deerjikist_form_record_invalid deerjikist, index
+    fields = { }
+
+    deerjikist.errors.each do |error|
+      field =
+          case error.attribute
+          when :platform, :code
+            "deerjikists.#{ index }.#{ error.attribute }"
+          else
+            :deerjikists
+          end
+
+      fields[field] ||= []
+      fields[field] << error.full_message
+    end
+
+    render_validation_error fields:
+  end
 end
diff --git a/backend/app/controllers/theatre_comments_controller.rb b/backend/app/controllers/theatre_comments_controller.rb
index 50ec9ef..fdc7ba1 100644
--- a/backend/app/controllers/theatre_comments_controller.rb
+++ b/backend/app/controllers/theatre_comments_controller.rb
@@ -15,7 +15,7 @@ class TheatreCommentsController < ApplicationController
     return head :unauthorized unless current_user
 
     content = params[:content]
-    return head :unprocessable_entity if content.blank?
+    return render_unprocessable_entity('本文は必須です．', field: :content) if content.blank?
 
     theatre = Theatre.find_by(id: params[:theatre_id])
     return head :not_found unless theatre
diff --git a/backend/app/controllers/users_controller.rb b/backend/app/controllers/users_controller.rb
index a144593..95ae677 100644
--- a/backend/app/controllers/users_controller.rb
+++ b/backend/app/controllers/users_controller.rb
@@ -42,12 +42,12 @@ class UsersController < ApplicationController
     return head :unauthorized if user&.id != params[:id].to_i
 
     name = params[:name]
-    return head :bad_request if name.blank?
+    return render_unprocessable_entity('名前は必須です．', field: :name) if name.blank?
 
     if user.update(name:)
       render json: user.slice(:id, :name, :inheritance_code, :role), status: :ok
     else
-      render json: user.errors, status: :unprocessable_entity
+      render_validation_error user
     end
   end
 
diff --git a/backend/app/controllers/wiki_pages_controller.rb b/backend/app/controllers/wiki_pages_controller.rb
index de42c52..818ae02 100644
--- a/backend/app/controllers/wiki_pages_controller.rb
+++ b/backend/app/controllers/wiki_pages_controller.rb
@@ -46,7 +46,7 @@ class WikiPagesController < ApplicationController
 
   def diff
     id = params[:id]
-    return head :bad_request if id.blank?
+    return render_bad_request('id は必須です．') if id.blank?
 
     from = params[:from].presence
     to = params[:to].presence
@@ -56,7 +56,7 @@ class WikiPagesController < ApplicationController
     from_rev = from && page.wiki_revisions.find(from)
     to_rev = to ? page.wiki_revisions.find(to) : page.current_revision
     if ((from_rev && !(from_rev.content?)) || !(to_rev&.content?))
-      return head :unprocessable_entity
+      return render_unprocessable_entity('差分を表示できない版です．')
     end
 
     diffs = Diff::LCS.sdiff(from_rev&.body&.lines || [], to_rev.body.lines)
@@ -89,7 +89,8 @@ class WikiPagesController < ApplicationController
     body = params[:body].to_s
     message = params[:message].presence
 
-    return head :unprocessable_entity if title.blank? || body.blank?
+    return render_unprocessable_entity('タイトルは必須です．', field: :title) if title.blank?
+    return render_unprocessable_entity('本文は必須です．', field: :body) if body.blank?
 
     tag_name = TagName.find_undiscard_or_create_by!(name: title)
 
@@ -101,8 +102,10 @@ class WikiPagesController < ApplicationController
             message:)
 
     render json: WikiPageRepr.base(page), status: :created
-  rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotUnique
-    head :unprocessable_entity
+  rescue ActiveRecord::RecordInvalid => e
+    render_validation_error e.record
+  rescue ActiveRecord::RecordNotUnique
+    render_record_not_unique
   end
 
   def update
@@ -112,7 +115,8 @@ class WikiPagesController < ApplicationController
     title = params[:title]&.strip
     body = params[:body].to_s
 
-    return head :unprocessable_entity if title.blank? || body.blank?
+    return render_unprocessable_entity('タイトルは必須です．', field: :title) if title.blank?
+    return render_unprocessable_entity('本文は必須です．', field: :body) if body.blank?
 
     page = WikiPage.find(params[:id])
     base_revision_id = params[:base_revision_id].presence
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index 4c3ddb1..2da8b02 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -94,7 +94,7 @@ class Post < ApplicationRecord
     return if !(f) || !(b)
 
     if f >= b
-      errors.add :original_created_before, 'オリジナルの作成日時の順番がをかしぃです．'
+      errors.add :original_created_at, 'オリジナルの作成日時の順番がをかしぃです．'
     end
   end
 
diff --git a/backend/app/representations/post_repr.rb b/backend/app/representations/post_repr.rb
index 87f59f9..e4f8198 100644
--- a/backend/app/representations/post_repr.rb
+++ b/backend/app/representations/post_repr.rb
@@ -2,20 +2,65 @@
 
 
 module PostRepr
-  BASE = { include: { tags: TagRepr::BASE, uploaded_user: UserRepr::BASE },
-           methods: [:parent_posts, :child_posts, :sibling_posts] }.freeze
+  BASE_FIELDS = [
+      :id,
+      :version_no,
+      :url,
+      :title,
+      :thumbnail_base,
+      :original_created_from,
+      :original_created_before,
+      :created_at,
+      :updated_at
+  ].freeze
 
   module_function
 
   def base post, current_user = nil
-    json = post.as_json(BASE)
-    return json.merge(viewed: false) unless current_user
+    json = common(post)
+    json['tags'] = tag_json(post.tags)
+    json['uploaded_user'] = post.uploaded_user && UserRepr.base(post.uploaded_user)
+    json['viewed'] = current_user ? current_user.viewed?(post) : false
+    json
+  end
 
-    viewed = current_user.viewed?(post)
-    json.merge(viewed:)
+  def detail post, current_user = nil, parent_posts: [], child_posts: [],
+             sibling_posts: { }, related: []
+    base(post, current_user).merge(
+        'parent_posts' => cards(parent_posts),
+        'child_posts' => cards(child_posts),
+        'sibling_posts' => sibling_posts.transform_keys(&:to_s).transform_values { |posts|
+          cards(posts)
+        },
+        'related' => cards(related))
+  end
+
+  def card post
+    common(post).merge('parent_posts' => [], 'child_posts' => [])
+  end
+
+  def cards posts
+    posts.map { |post| card(post) }
   end
 
   def many posts, current_user = nil
     posts.map { |p| base(p, current_user) }
   end
+
+  def common post
+    BASE_FIELDS.to_h { |field| [field.to_s, post.public_send(field)] }
+               .merge('thumbnail' => thumbnail_url(post))
+  end
+
+  def tag_json tags
+    tags.map { |tag| TagRepr.inline(tag) }
+  end
+
+  def thumbnail_url post
+    return nil unless post.thumbnail.attached?
+
+    Rails.application.routes.url_helpers.rails_blob_url(post.thumbnail, only_path: false)
+  rescue
+    nil
+  end
 end
diff --git a/backend/app/representations/tag_repr.rb b/backend/app/representations/tag_repr.rb
index 705c096..28be332 100644
--- a/backend/app/representations/tag_repr.rb
+++ b/backend/app/representations/tag_repr.rb
@@ -12,5 +12,9 @@ module TagRepr
                             parents: tag.parents.map { _1.as_json(BASE) })
   end
 
+  def inline tag
+    tag.as_json(BASE).merge(aliases: [], parents: [])
+  end
+
   def many(tags) = tags.map { |t| base(t) }
 end
diff --git a/backend/spec/requests/error_responses_spec.rb b/backend/spec/requests/error_responses_spec.rb
new file mode 100644
index 0000000..a7b5191
--- /dev/null
+++ b/backend/spec/requests/error_responses_spec.rb
@@ -0,0 +1,47 @@
+require 'rails_helper'
+
+RSpec.describe 'error responses', type: :request do
+  describe 'manual input errors' do
+    it 'returns a stable payload for bad requests' do
+      get '/tags/name/%20/deerjikists'
+
+      expect(response).to have_http_status(:bad_request)
+      expect(json).to include(
+          'type' => 'bad_request',
+          'message' => be_present,
+          'errors' => {},
+          'base_errors' => [be_present])
+    end
+
+    it 'returns a stable field-error payload for unprocessable requests' do
+      member = create(:user, :member)
+      tag = create(:tag, :general, name: 'error_response_tag')
+      sign_in_as(member)
+
+      patch "/tags/#{ tag.id }", params: { category: 'nico' }
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json).to include(
+          'type' => 'validation_error',
+          'message' => '入力内容を確認してください．',
+          'base_errors' => [])
+      expect(json.fetch('errors')).to include(
+          'category' => ['ニコタグは変更できません．'])
+    end
+  end
+
+  describe 'model validation errors' do
+    it 'returns field messages for model errors' do
+      user = create(:user)
+      sign_in_as(user)
+
+      put "/users/#{ user.id }", params: { name: 'a' * 256 }
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json).to include(
+          'type' => 'validation_error',
+          'message' => '入力内容を確認してください．')
+      expect(json.fetch('errors').fetch('name')).to include(be_present)
+    end
+  end
+end
diff --git a/backend/spec/requests/materials_spec.rb b/backend/spec/requests/materials_spec.rb
index f2cc27e..d4085ec 100644
--- a/backend/spec/requests/materials_spec.rb
+++ b/backend/spec/requests/materials_spec.rb
@@ -141,16 +141,21 @@ RSpec.describe 'Materials API', type: :request do
     context 'when logged in' do
       before { sign_in_as(guest_user) }
 
-      it 'returns 400 when tag is blank' do
+      it 'returns 422 when tag is blank' do
         post '/materials', params: { tag: '   ', file: dummy_upload }
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to include(
+            'tag' => ['タグは必須です．'])
       end
 
-      it 'returns 400 when both file and url are blank' do
+      it 'returns 422 when both file and url are blank' do
         post '/materials', params: { tag: 'material_create_blank' }
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to include(
+            'file' => ['ファイルまたは URL は必須です．'],
+            'url' => ['ファイルまたは URL は必須です．'])
       end
 
       it 'creates a material with an attached file' do
@@ -261,21 +266,26 @@ RSpec.describe 'Materials API', type: :request do
         expect(response).to have_http_status(:not_found)
       end
 
-      it 'returns 400 when tag is blank' do
+      it 'returns 422 when tag is blank' do
         put "/materials/#{ material.id }", params: {
           tag: '   ',
           file: dummy_upload
         }
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to include(
+            'tag' => ['タグは必須です．'])
       end
 
-      it 'returns 400 when both file and url are blank' do
+      it 'returns 422 when both file and url are blank' do
         put "/materials/#{ material.id }", params: {
           tag: 'material_update_no_payload'
         }
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to include(
+            'file' => ['ファイルまたは URL は必須です．'],
+            'url' => ['ファイルまたは URL は必須です．'])
       end
 
       it 'updates tag, url, file, and updated_by_user' do
diff --git a/backend/spec/requests/nico_tags_spec.rb b/backend/spec/requests/nico_tags_spec.rb
index 26d5de0..b9ac6c6 100644
--- a/backend/spec/requests/nico_tags_spec.rb
+++ b/backend/spec/requests/nico_tags_spec.rb
@@ -3,12 +3,68 @@ require 'rails_helper'
 
 RSpec.describe 'NicoTags', type: :request do
   describe 'GET /tags/nico' do
-    it 'returns tags and next_cursor when overflowing limit' do
-      create_list(:tag, 21, :nico)
-      get '/tags/nico', params: { limit: 20 }
+    it 'returns paginated tags and total count' do
+      create_list(:tag, 3, :nico)
+
+      get '/tags/nico', params: { page: 2, limit: 2 }
+
       expect(response).to have_http_status(:ok)
-      expect(json['tags'].size).to eq(20)
-      expect(json['next_cursor']).to be_present
+      expect(json['tags'].size).to eq(1)
+      expect(json['count']).to eq(3)
+    end
+
+    it 'filters by nico tag name, linked tag name, and link status' do
+      linked = create(:tag, :nico)
+      linked.tag_name.update!(name: 'nico:search_linked')
+      unlinked = create(:tag, :nico)
+      unlinked.tag_name.update!(name: 'nico:search_unlinked')
+      other = create(:tag, :nico)
+      other.tag_name.update!(name: 'nico:other')
+      destination = create(:tag, :general)
+      destination.tag_name.update!(name: 'destination_search')
+      NicoTagRelation.create!(nico_tag: linked, tag: destination)
+      NicoTagRelation.create!(nico_tag: other, tag: create(:tag, :general))
+
+      get '/tags/nico', params: {
+        name: 'search_',
+        linked_tag: 'destination_',
+        link_status: 'linked'
+      }
+
+      expect(response).to have_http_status(:ok)
+      expect(json['count']).to eq(1)
+      expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([linked.id])
+
+      get '/tags/nico', params: { name: 'search_', link_status: 'unlinked' }
+
+      expect(json['count']).to eq(1)
+      expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([unlinked.id])
+    end
+
+    it 'sorts by name and timestamps' do
+      older = create(:tag, :nico)
+      older.tag_name.update!(name: 'nico:a')
+      older.update_columns(created_at: 2.days.ago)
+      newer = create(:tag, :nico)
+      newer.tag_name.update!(name: 'nico:b')
+      newer.update_columns(created_at: 1.day.ago)
+      older_post_tag =
+        PostTag.create!(post: Post.create!(url: 'https://example.com/nico-older'), tag: older)
+      older_post_tag.update_columns(created_at: 1.hour.ago)
+      newer_post_tag =
+        PostTag.create!(post: Post.create!(url: 'https://example.com/nico-newer'), tag: newer)
+      newer_post_tag.update_columns(created_at: 2.hours.ago)
+
+      get '/tags/nico', params: { order: 'name:desc' }
+      expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([newer.id, older.id])
+
+      get '/tags/nico', params: { order: 'created_at:asc' }
+      expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([older.id, newer.id])
+
+      get '/tags/nico', params: { order: 'updated_at:desc' }
+      expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([older.id, newer.id])
+      expect(Time.zone.parse(json.fetch('tags').first.fetch('recent_post_tag_created_at')))
+        .to be_within(1.second).of(older_post_tag.created_at)
     end
   end
 
@@ -75,7 +131,7 @@ RSpec.describe 'NicoTags', type: :request do
       expect(versions.last.created_by_user_id).to eq(admin.id)
     end
 
-    it '400 when linked tag normalises to nico tag' do
+    it 'returns 422 when linked tag normalises to nico tag' do
       sign_in_as(member)
 
       other_nico = create(:tag, :nico, name: 'nico:linked_ng')
@@ -87,7 +143,37 @@ RSpec.describe 'NicoTags', type: :request do
         patch "/tags/nico/#{nico_tag.id}", params: { tags: 'linked_ng_alias' }
       }.not_to change(NicoTagVersion, :count)
 
-      expect(response).to have_http_status(:bad_request)
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors')).to include(
+          'tags' => ['ニコニコ・タグ同士は連携できません．'])
+    end
+
+    it 'returns the tags field error when a nico tag is specified directly' do
+      sign_in_as(member)
+
+      patch "/tags/nico/#{nico_tag.id}", params: { tags: 'nico:linked_ng' }
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors')).to include(
+          'tags' => ['ニコニコ・タグ同士は連携できません．'])
+    end
+
+    it 'returns tag name validation errors on the tags field and rolls back created tags' do
+      sign_in_as(member)
+      TagNameSanitisationRule.create!(
+        priority: 1,
+        source_pattern: 'invalid',
+        replacement: 'valid'
+      )
+      nico_tag
+
+      expect {
+        patch "/tags/nico/#{nico_tag.id}", params: { tags: 'created_first invalid' }
+      }.not_to change(TagName, :count)
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors').fetch('tags')).to include(
+          a_string_including('タグ名 “invalid”:', '名前に使用できない文字が含まれてゐます．'))
     end
   end
 end
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index 14b51e7..2914220 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -57,6 +57,23 @@ RSpec.describe 'Posts API', type: :request do
     post_write_params({ base_version_no: base_version.version_no }.merge(params))
   end
 
+  def count_sql_queries
+    count = 0
+
+    callback = lambda do |_name, _started, _finished, _id, payload|
+      next if payload[:cached]
+      next if ['SCHEMA', 'TRANSACTION'].include?(payload[:name])
+
+      count += 1
+    end
+
+    ActiveSupport::Notifications.subscribed(callback, 'sql.active_record') do
+      yield
+    end
+
+    count
+  end
+
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
   let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
 
@@ -558,6 +575,59 @@ RSpec.describe 'Posts API', type: :request do
           expect(sibling_ids).to include(sibling_post.id)
         end
       end
+
+      it 'does not issue a query per tag or related post' do
+        user = create_member_user!
+
+        tags =
+            15.times.map do |i|
+              tag_name = TagName.create!(name: "show_query_tag_#{ i }")
+              tag = Tag.create!(tag_name:, category: :general)
+              TagName.create!(name: "show_query_alias_#{ i }", canonical: tag_name)
+              PostTag.create!(post: post_record, tag:)
+              tag
+            end
+
+        tags.each_cons(2) do |parent_tag, child_tag|
+          TagImplication.create!(parent_tag:, tag: child_tag)
+        end
+
+        parent_post = Post.create!(
+          title: 'query parent post',
+          url: 'https://example.com/query-parent-post'
+        )
+        sibling_post = Post.create!(
+          title: 'query sibling post',
+          url: 'https://example.com/query-sibling-post'
+        )
+        child_post = Post.create!(
+          title: 'query child post',
+          url: 'https://example.com/query-child-post'
+        )
+
+        PostImplication.create!(post: post_record, parent_post:)
+        PostImplication.create!(post: sibling_post, parent_post:)
+        PostImplication.create!(post: child_post, parent_post: post_record)
+
+        20.times do |i|
+          related_post = Post.create!(
+            title: "query related post #{ i }",
+            url: "https://example.com/query-related-post-#{ i }"
+          )
+          PostSimilarity.create!(post: post_record,
+                                 target_post: related_post,
+                                 cos: 1.0 - (i / 100.0))
+        end
+
+        query_count =
+            count_sql_queries do
+              get "/posts/#{ post_record.id }",
+                  headers: { 'X-Transfer-Code' => user.inheritance_code }
+            end
+
+        expect(response).to have_http_status(:ok)
+        expect(query_count).to be <= 45
+      end
     end
 
     context 'when post does not exist' do
@@ -634,7 +704,7 @@ RSpec.describe 'Posts API', type: :request do
             category: :nico)
       end
 
-      it 'return 400' do
+      it 'returns 422 with tag field errors' do
         sign_in_as(member)
 
         post '/posts', params: post_write_params(
@@ -644,7 +714,13 @@ RSpec.describe 'Posts API', type: :request do
           thumbnail: dummy_upload
         )
 
-        expect(response).to have_http_status(:bad_request), response.body
+        expect(response).to have_http_status(:unprocessable_entity), response.body
+        expect(json).to include(
+            'type' => 'validation_error',
+            'message' => '入力内容を確認してください．',
+            'base_errors' => [])
+        expect(json.fetch('errors')).to include(
+            'tags' => ['ニコニコ・タグは直接指定できません．'])
       end
     end
 
@@ -861,7 +937,7 @@ RSpec.describe 'Posts API', type: :request do
             category: :nico)
       end
 
-      it 'return 400' do
+      it 'returns 422 with tag field errors' do
         sign_in_as(member)
 
         put "/posts/#{post_record.id}", params: post_update_params(
@@ -869,7 +945,13 @@ RSpec.describe 'Posts API', type: :request do
             title: 'updated title',
             tags: 'nico:nico_tag')
 
-        expect(response).to have_http_status(:bad_request), response.body
+        expect(response).to have_http_status(:unprocessable_entity), response.body
+        expect(json).to include(
+            'type' => 'validation_error',
+            'message' => '入力内容を確認してください．',
+            'base_errors' => [])
+        expect(json.fetch('errors')).to include(
+            'tags' => ['ニコニコ・タグは直接指定できません．'])
       end
     end
 
diff --git a/backend/spec/requests/tags_deerjikists_spec.rb b/backend/spec/requests/tags_deerjikists_spec.rb
index c599f17..7bbf224 100644
--- a/backend/spec/requests/tags_deerjikists_spec.rb
+++ b/backend/spec/requests/tags_deerjikists_spec.rb
@@ -275,6 +275,30 @@ RSpec.describe 'Tags deerjikists API', type: :request do
         end
       end
 
+      context 'when a row is invalid' do
+        let(:payload) do
+          [
+            { platform: '', code: code1 },
+          ]
+        end
+
+        it 'returns 422 with indexed field errors and does not replace existing deerjikists' do
+          Deerjikist.create!(platform: platform1, code: code1, tag: tag)
+
+          expect {
+            do_request
+          }.not_to change { Deerjikist.where(tag: tag).map { |d| [d.platform, d.code] } }
+
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(json).to include(
+              'type' => 'validation_error',
+              'message' => '入力内容を確認してください．',
+              'base_errors' => [])
+          expect(json.fetch('errors')).to include(
+              'deerjikists.0.platform' => [be_present])
+        end
+      end
+
       context 'when youtube code is handle' do
         let(:channel_id) { 'UCabcdefghijklmnopqrstuv' }
         let(:payload) do
diff --git a/backend/spec/requests/users_spec.rb b/backend/spec/requests/users_spec.rb
index 67c556f..c58f614 100644
--- a/backend/spec/requests/users_spec.rb
+++ b/backend/spec/requests/users_spec.rb
@@ -90,12 +90,14 @@ RSpec.describe 'Users', type: :request do
       expect(response).to have_http_status(:unauthorized)
     end
 
-    it 'returns 400 when name is blank' do
+    it 'returns 422 when name is blank' do
       put "/users/#{user.id}",
           params: { name: '   ' },
           headers: auth_headers(user)
 
-      expect(response).to have_http_status(:bad_request)
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors')).to include(
+          'name' => ['名前は必須です．'])
     end
 
     it 'updates name and returns user slice' do
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 2eeaf79..784fbf8 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -62,6 +62,7 @@ const RouteTransitionWrapper = ({ user, setUser }: {
 	  <Route path="/tags/:id" element={<TagDetailPage/>}/>
 	  <Route path="/tags/:id/deerjikists" element={<DeerjikistDetailPage/>}/>
 	  <Route path="/tags/nico" element={<NicoTagListPage user={user}/>}/>
+	  <Route path="/nico/tags" element={<NicoTagListPage user={user}/>}/>
 	  <Route path="/tags/changes" element={<TagHistoryPage/>}/>
 	  <Route path="/theatres/:id" element={<TheatreDetailPage/>}/>
 	  <Route path="/materials" element={<MaterialBasePage/>}>
@@ -158,4 +159,4 @@ const App: FC = () => {
       </>)
 }
 
-export default App
\ No newline at end of file
+export default App
diff --git a/frontend/src/components/PostEditForm.tsx b/frontend/src/components/PostEditForm.tsx
index bc1048e..b17fabc 100644
--- a/frontend/src/components/PostEditForm.tsx
+++ b/frontend/src/components/PostEditForm.tsx
@@ -2,17 +2,23 @@ import { useEffect, useState } from 'react'
 
 import PostFormTagsArea from '@/components/PostFormTagsArea'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
-import Label from '@/components/common/Label'
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import { useDialogue } from '@/components/dialogues/DialogueProvider'
 import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { isApiError } from '@/lib/api'
 import { updatePost } from '@/lib/posts'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC, FormEvent } from 'react'
 
 import type { Post, Tag } from '@/types'
 
+type PostFormField =
+    'parentPostIds' | 'tags' | 'originalCreatedAt'
+
 
 const tagsToStr = (tags: Tag[]): string => {
   const result: Tag[] = []
@@ -35,6 +41,8 @@ type Props = { post:    Post
 
 const PostEditForm: FC<Props> = ({ post, onSave }) => {
   const [disabled, setDisabled] = useState (false)
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<PostFormField> ()
   const [originalCreatedBefore, setOriginalCreatedBefore] =
       useState<string | null> (post.originalCreatedBefore)
   const [originalCreatedFrom, setOriginalCreatedFrom] =
@@ -47,6 +55,8 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
   const dialogue = useDialogue ()
 
   const update = async (...args: Parameters<typeof updatePost>) => {
+    clearValidationErrors ()
+
     try
     {
       const data = await updatePost (...args)
@@ -67,7 +77,14 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
 
       if (response?.status !== 409)
 	{
-	  toast ({ description: '更新はできなかったよ……' })
+	  if (applyValidationError (e))
+	    {
+	      toast ({ description: '更新はできなかったよ……' })
+	      return
+	    }
+
+	  toast ({ title: '失敗……', description: '入力を確認してください．' })
+
 	  return
 	}
 
@@ -122,33 +139,38 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
 
   return (
       <form onSubmit={handleSubmit} className="max-w-xl pt-2 space-y-4">
+	<FieldError messages={baseErrors}/>
+
 	{/* タイトル */}
-	<div>
-	  <Label>タイトル</Label>
-	  <input
-	      type="text"
-	      disabled={disabled}
-	      className="w-full border rounded p-2"
-	      value={title ?? ''}
-	      onChange={ev => setTitle (ev.target.value)}/>
-	</div>
+	<FormField label="タイトル">
+	  {({ invalid }) => (
+	      <input
+		  type="text"
+		  disabled={disabled}
+		  className={inputClass (invalid)}
+		  value={title ?? ''}
+		  onChange={ev => setTitle (ev.target.value)}/>)}
+	</FormField>
 
 	{/* 親投稿 */}
-	<div>
-	  <Label>親投稿</Label>
-	  <input
-	      type="text"
-	      disabled={disabled}
-	      value={parentPostIds}
-	      onChange={e => setParentPostIds (e.target.value)}
-	      className="w-full border p-2 rounded"/>
-	</div>
+	<FormField label="親投稿" messages={fieldErrors.parentPostIds}>
+	  {({ describedBy, invalid }) => (
+	      <input
+		  type="text"
+		  disabled={disabled}
+		  value={parentPostIds}
+		  onChange={e => setParentPostIds (e.target.value)}
+		  aria-describedby={describedBy}
+		  aria-invalid={invalid}
+		  className={inputClass (invalid)}/>)}
+	</FormField>
 
 	{/* タグ */}
 	<PostFormTagsArea
 	    disabled={disabled}
 	    tags={tags}
-	    setTags={setTags}/>
+	    setTags={setTags}
+	    errors={fieldErrors.tags}/>
 
 	{/* オリジナルの作成日時 */}
 	<PostOriginalCreatedTimeField
@@ -156,15 +178,14 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
 	    originalCreatedFrom={originalCreatedFrom}
 	    setOriginalCreatedFrom={setOriginalCreatedFrom}
 	    originalCreatedBefore={originalCreatedBefore}
-	    setOriginalCreatedBefore={setOriginalCreatedBefore}/>
+	    setOriginalCreatedBefore={setOriginalCreatedBefore}
+	    errors={fieldErrors.originalCreatedAt}/>
 
 	{/* 送信 */}
-	<Button
-	    type="submit"
-	    disabled={disabled}>
+	<Button type="submit" disabled={disabled}>
 	  更新
 	</Button>
       </form>)
 }
 
-export default PostEditForm
\ No newline at end of file
+export default PostEditForm
diff --git a/frontend/src/components/PostFormTagsArea.tsx b/frontend/src/components/PostFormTagsArea.tsx
index 513af9d..19c1c6a 100644
--- a/frontend/src/components/PostFormTagsArea.tsx
+++ b/frontend/src/components/PostFormTagsArea.tsx
@@ -3,7 +3,7 @@
 import { useRef, useState } from 'react'
 
 import TagSearchBox from '@/components/TagSearchBox'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import TextArea from '@/components/common/TextArea'
 import { apiGet } from '@/lib/api'
 
@@ -33,10 +33,11 @@ const replaceToken = (value: string, start: number, end: number, text: string) =
 
 type Props = Omit<ComponentPropsWithoutRef<'textarea'>, 'value' | 'onChange' | 'onBlur'> & {
     tags:     string
-    setTags:  (tags: string) => void }
+    setTags:  (tags: string) => void
+    errors?:  string[] }
 
 
-const PostFormTagsArea: FC<Props> = ({ tags, setTags, ...rest }) => {
+const PostFormTagsArea: FC<Props> = ({ tags, setTags, errors, ...rest }) => {
   const ref = useRef<HTMLTextAreaElement> (null)
 
   const [bounds, setBounds] = useState<{ start: number; end: number }> ({ start: 0, end: 0 })
@@ -73,30 +74,34 @@ const PostFormTagsArea: FC<Props> = ({ tags, setTags, ...rest }) => {
   }
 
   return (
-      <div className="relative w-full">
-	<Label>タグ</Label>
-	<TextArea
-	    {...rest}
-	    ref={ref}
-	    value={tags}
-	    onChange={ev => setTags (ev.target.value)}
-	    onSelect={async (ev: SyntheticEvent<HTMLTextAreaElement>) => {
-	      const pos = (ev.target as HTMLTextAreaElement).selectionStart
-	      await recompute (pos)
-	    }}
-	    onFocus={() => setFocused (true)}
-	    onBlur={() => {
-	      setFocused (false)
-	      setSuggestionsVsbl (false)
-	    }}/>
-	{focused && (
-	    <TagSearchBox
-		suggestions={suggestionsVsbl && suggestions.length > 0
-			     ? suggestions
-			     : [] as Tag[]}
-		activeIndex={-1}
-		onSelect={handleTagSelect}/>)}
-      </div>)
+      <FormField className="relative w-full" label="タグ" messages={errors}>
+	{({ describedBy, invalid }) => (
+	    <>
+	      <TextArea
+		  {...rest}
+		  ref={ref}
+		  value={tags}
+		  aria-describedby={describedBy}
+		  invalid={invalid}
+		  onChange={ev => setTags (ev.target.value)}
+		  onSelect={async (ev: SyntheticEvent<HTMLTextAreaElement>) => {
+		    const pos = (ev.target as HTMLTextAreaElement).selectionStart
+		    await recompute (pos)
+		  }}
+		  onFocus={() => setFocused (true)}
+		  onBlur={() => {
+		    setFocused (false)
+		    setSuggestionsVsbl (false)
+		  }}/>
+	      {focused && (
+		  <TagSearchBox
+		      suggestions={suggestionsVsbl && suggestions.length > 0
+				   ? suggestions
+				   : [] as Tag[]}
+		      activeIndex={-1}
+		      onSelect={handleTagSelect}/>)}
+	    </>)}
+      </FormField>)
 }
 
-export default PostFormTagsArea
\ No newline at end of file
+export default PostFormTagsArea
diff --git a/frontend/src/components/PostOriginalCreatedTimeField.tsx b/frontend/src/components/PostOriginalCreatedTimeField.tsx
index 625b5a3..2ddbee4 100644
--- a/frontend/src/components/PostOriginalCreatedTimeField.tsx
+++ b/frontend/src/components/PostOriginalCreatedTimeField.tsx
@@ -1,5 +1,5 @@
 import DateTimeField from '@/components/common/DateTimeField'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import { Button } from '@/components/ui/button'
 
 import type { FC } from 'react'
@@ -9,68 +9,81 @@ type Props = {
     originalCreatedFrom:      string | null
     setOriginalCreatedFrom:   (x: string | null) => void
     originalCreatedBefore:    string | null
-    setOriginalCreatedBefore: (x: string | null) => void }
+    setOriginalCreatedBefore: (x: string | null) => void
+    errors?:                  string[] }
 
 
-const PostOriginalCreatedTimeField: FC<Props> = ({ disabled,
-		   originalCreatedFrom,
-		   setOriginalCreatedFrom,
-		   originalCreatedBefore,
-		   setOriginalCreatedBefore }) => (
-    <div>
-      <Label>オリジナルの作成日時</Label>
-      <div className="my-1 flex">
-	<div className="w-80">
-	  <DateTimeField
-	      className="mr-2"
-	      disabled={disabled ?? false}
-	      value={originalCreatedFrom ?? undefined}
-	      onChange={setOriginalCreatedFrom}
-	      onBlur={ev => {
-		const v = ev.target.value
-		if (!(v))
-		  return
+const PostOriginalCreatedTimeField: FC<Props> = (
+    { disabled,
+      originalCreatedFrom,
+      setOriginalCreatedFrom,
+      originalCreatedBefore,
+      setOriginalCreatedBefore,
+      errors }: Props,
+) => (
+    <FormField label="オリジナルの作成日時" messages={errors}>
+      {({ describedBy, invalid }) => (
+	  <>
+	    <div className="my-1 flex">
+	      <div className="w-80">
+		<DateTimeField
+		    className="mr-2"
+		    disabled={disabled ?? false}
+		    aria-describedby={describedBy}
+		    aria-invalid={invalid}
+		    invalid={invalid}
+		    value={originalCreatedFrom ?? undefined}
+		    onChange={setOriginalCreatedFrom}
+		    onBlur={ev => {
+		      const v = ev.target.value
+		      if (!(v))
+			return
 
-		const d = new Date (v)
-		if (d.getMinutes () === 0 && d.getHours () === 0)
-		  d.setDate (d.getDate () + 1)
-		else
-		  d.setMinutes (d.getMinutes () + 1)
-		setOriginalCreatedBefore (d.toISOString ())
-	      }}/>
-	  以降
-	</div>
-	<div>
-	  <Button
-	      className="bg-gray-600 text-white rounded"
-	      disabled={disabled}
-	      onClick={() => {
-		setOriginalCreatedFrom (null)
-	      }}>
-	    リセット
-	  </Button>
-	</div>
-      </div>
-      <div className="my-1 flex">
-	<div className="w-80">
-	  <DateTimeField
-	      className="mr-2"
-	      disabled={disabled}
-	      value={originalCreatedBefore ?? undefined}
-	      onChange={setOriginalCreatedBefore}/>
-	  より前
-	</div>
-	<div>
-	  <Button
-	      className="bg-gray-600 text-white rounded"
-	      disabled={disabled}
-	      onClick={() => {
-		setOriginalCreatedBefore (null)
-	      }}>
-	    リセット
-	  </Button>
-	</div>
-      </div>
-    </div>)
+		      const d = new Date (v)
+		      if (d.getMinutes () === 0 && d.getHours () === 0)
+			d.setDate (d.getDate () + 1)
+		      else
+			d.setMinutes (d.getMinutes () + 1)
+		      setOriginalCreatedBefore (d.toISOString ())
+		    }}/>
+		以降
+	      </div>
+	      <div>
+		<Button
+		    className="bg-gray-600 text-white rounded"
+		    disabled={disabled}
+		    onClick={() => {
+		      setOriginalCreatedFrom (null)
+		    }}>
+		  リセット
+		</Button>
+	      </div>
+	    </div>
 
-export default PostOriginalCreatedTimeField
\ No newline at end of file
+	    <div className="my-1 flex">
+	      <div className="w-80">
+		<DateTimeField
+		    className="mr-2"
+		    disabled={disabled}
+		    aria-describedby={describedBy}
+		    aria-invalid={invalid}
+		    invalid={invalid}
+		    value={originalCreatedBefore ?? undefined}
+		    onChange={setOriginalCreatedBefore}/>
+		より前
+	      </div>
+	      <div>
+		<Button
+		    className="bg-gray-600 text-white rounded"
+		    disabled={disabled}
+		    onClick={() => {
+		      setOriginalCreatedBefore (null)
+		    }}>
+		  リセット
+		</Button>
+	      </div>
+	    </div>
+	  </>)}
+    </FormField>)
+
+export default PostOriginalCreatedTimeField
diff --git a/frontend/src/components/TagSearch.tsx b/frontend/src/components/TagSearch.tsx
index e9833f1..9447e1f 100644
--- a/frontend/src/components/TagSearch.tsx
+++ b/frontend/src/components/TagSearch.tsx
@@ -4,6 +4,7 @@ import { useEffect, useState } from 'react'
 import { useNavigate, useLocation } from 'react-router-dom'
 
 import { apiGet } from '@/lib/api'
+import { inputClass } from '@/lib/utils'
 
 import TagSearchBox from './TagSearchBox'
 
@@ -110,11 +111,12 @@ const TagSearch: FC = () => {
 	       onFocus={() => setSuggestionsVsbl (true)}
 	       onBlur={() => setSuggestionsVsbl (false)}
 	       onKeyDown={handleKeyDown}
-	       className="w-full px-3 py-2 border rounded dark:border-gray-600 dark:bg-gray-800 dark:text-white"/>
+	       className={inputClass (false,
+				      'px-3 py-2 dark:border-gray-600 dark:bg-gray-800 dark:text-white')}/>
 	<TagSearchBox suggestions={suggestionsVsbl && suggestions.length ? suggestions : [] as Tag[]}
 		      activeIndex={activeIndex}
 		      onSelect={handleTagSelect}/>
       </div>)
 }
 
-export default TagSearch
\ No newline at end of file
+export default TagSearch
diff --git a/frontend/src/components/common/DateTimeField.tsx b/frontend/src/components/common/DateTimeField.tsx
index 9e50595..0cb2210 100644
--- a/frontend/src/components/common/DateTimeField.tsx
+++ b/frontend/src/components/common/DateTimeField.tsx
@@ -22,10 +22,11 @@ type Props = Omit<ComponentPropsWithoutRef<'input'>, 'onChange'> & {
     value?:     string
     onChange?:  (isoUTC: string | null) => void
     className?: string
-    onBlur?:    (ev: FocusEvent<HTMLInputElement>) => void }
+    onBlur?:    (ev: FocusEvent<HTMLInputElement>) => void
+    invalid?:   boolean }
 
 
-const DateTimeField: FC<Props> = ({ value, onChange, className, onBlur, ...rest }) => {
+const DateTimeField: FC<Props> = ({ value, onChange, className, onBlur, invalid, ...rest }) => {
   const [local, setLocal] = useState ('')
 
   useEffect (() => {
@@ -35,9 +36,19 @@ const DateTimeField: FC<Props> = ({ value, onChange, className, onBlur, ...rest
   return (
       <input
 	  {...rest}
-	  className={cn ('border rounded p-2', className)}
+	  className={cn ('border rounded p-2',
+			 (invalid
+			  ? ['border-red-500 bg-red-50 text-red-900',
+			     'focus:border-red-500 focus:outline-none',
+			     'focus:ring-2 focus:ring-red-200',
+			     'dark:border-red-500 dark:bg-red-950/30 dark:text-red-100']
+			  : ['border-gray-300',
+			     'focus:border-blue-500 focus:outline-none',
+			     'focus:ring-2 focus:ring-blue-200']),
+			 className)}
 	  type="datetime-local"
 	  value={local}
+	  aria-invalid={invalid}
 	  onChange={ev => {
 	    const v = ev.target.value
 	    setLocal (v)
@@ -46,4 +57,4 @@ const DateTimeField: FC<Props> = ({ value, onChange, className, onBlur, ...rest
 	  onBlur={onBlur}/>)
 }
 
-export default DateTimeField
\ No newline at end of file
+export default DateTimeField
diff --git a/frontend/src/components/common/FieldError.tsx b/frontend/src/components/common/FieldError.tsx
new file mode 100644
index 0000000..01d790d
--- /dev/null
+++ b/frontend/src/components/common/FieldError.tsx
@@ -0,0 +1,18 @@
+import type { FC } from 'react'
+
+type Props = { id?:        string
+	       messages?:  string[] }
+
+
+export const FieldError: FC<Props> = ({ id, messages }: Props) => {
+  if (!(messages) || messages.length === 0)
+    return null
+
+  return (
+      <ul id={id} className="mt-1 space-y-1 text-red-700 dark:text-red-300">
+	{messages.map ((message, i) => <li key={i}>{message}</li>)}
+      </ul>)
+}
+
+
+export default FieldError
diff --git a/frontend/src/components/common/FormField.tsx b/frontend/src/components/common/FormField.tsx
new file mode 100644
index 0000000..67b1f65
--- /dev/null
+++ b/frontend/src/components/common/FormField.tsx
@@ -0,0 +1,36 @@
+import { useId } from 'react'
+
+import FieldError from '@/components/common/FieldError'
+import Label from '@/components/common/Label'
+import { cn } from '@/lib/utils'
+
+import type { FC, ReactNode } from 'react'
+
+type FieldState = { describedBy?: string
+		    invalid:      boolean }
+
+type Props = {
+    children:   (state: FieldState) => ReactNode
+    checkBox?:   { label:    string
+		   checked:  boolean
+		   onChange: (event: React.ChangeEvent<HTMLInputElement>) => void }
+    className?:  string
+    label:      ReactNode
+    messages?:  string[] }
+
+
+const FormField: FC<Props> = ({ children, checkBox, className, label, messages }: Props) => {
+  const id = useId ()
+  const invalid = messages != null && messages.length > 0
+  const errorId = invalid ? `${ id }-error` : undefined
+
+  return (
+      <div className={cn (className)}>
+	<Label checkBox={checkBox} invalid={invalid}>{label}</Label>
+	{children ({ describedBy: errorId, invalid })}
+	<FieldError id={errorId} messages={messages}/>
+      </div>)
+}
+
+
+export default FormField
diff --git a/frontend/src/components/common/Label.tsx b/frontend/src/components/common/Label.tsx
index 55a3978..d039115 100644
--- a/frontend/src/components/common/Label.tsx
+++ b/frontend/src/components/common/Label.tsx
@@ -1,32 +1,39 @@
 import React from 'react'
 
+import { cn } from '@/lib/utils'
+
 import type { FC } from 'react'
 
 type Props = { children:  React.ReactNode
-               checkBox?: { label:    string
-                            checked:  boolean
-                            onChange: (event: React.ChangeEvent<HTMLInputElement>) => void } }
+	       checkBox?: { label:    string
+			    checked:  boolean
+			    onChange: (event: React.ChangeEvent<HTMLInputElement>) => void }
+	       invalid?:  boolean }
 
 
-const Label: FC<Props> = ({ children, checkBox }) => {
+const Label: FC<Props> = ({ children, checkBox, invalid }: Props) => {
+  const labelClassName = cn ('block font-semibold mb-1',
+			     invalid && 'text-red-700 dark:text-red-300')
+
   if (!(checkBox))
     {
       return (
-          <label className="block font-semibold mb-1">
-            {children}
-          </label>)
+	  <label className={labelClassName}>
+	    {children}
+	  </label>)
     }
 
   return (
       <div className="flex gap-2 mb-1">
-        <label className="flex-1 block font-semibold">{children}</label>
-        <label className="flex items-center block gap-1">
-          <input type="checkbox"
-                 checked={checkBox.checked}
-                 onChange={checkBox.onChange}/>
-          {checkBox.label}
-        </label>
+	<label className="flex-1 block font-semibold">{children}</label>
+	<label className="flex items-center block gap-1">
+	  <input type="checkbox"
+		 checked={checkBox.checked}
+		 onChange={checkBox.onChange}/>
+	  {checkBox.label}
+	</label>
       </div>)
 }
 
+
 export default Label
diff --git a/frontend/src/components/common/TagInput.tsx b/frontend/src/components/common/TagInput.tsx
index dad1ce8..d0d6491 100644
--- a/frontend/src/components/common/TagInput.tsx
+++ b/frontend/src/components/common/TagInput.tsx
@@ -2,6 +2,7 @@ import { useState } from 'react'
 
 import TagSearchBox from '@/components/TagSearchBox'
 import { apiGet } from '@/lib/api'
+import { inputClass } from '@/lib/utils'
 
 import type { FC, ChangeEvent, KeyboardEvent } from 'react'
 
@@ -9,10 +10,13 @@ import type { Tag } from '@/types'
 
 
 type Props = {
-    value:      string
-    setValue:   (value: string) => void }
+    describedBy?:  string
+    invalid?:      boolean
+    value:         string
+    setValue:      (value: string) => void }
 
-const TagInput: FC<Props> = ({ value, setValue }) => {
+
+const TagInput: FC<Props> = ({ describedBy, invalid, value, setValue }) => {
   const [activeIndex, setActiveIndex] = useState (-1)
   const [suggestions, setSuggestions] = useState<Tag[]> ([])
   const [suggestionsVsbl, setSuggestionsVsbl] = useState (false)
@@ -85,12 +89,14 @@ const TagInput: FC<Props> = ({ value, setValue }) => {
       <div className="relative">
 	<input
 	    type="text"
+	    aria-describedby={describedBy}
+	    aria-invalid={invalid}
 	    value={value}
 	    onChange={whenChanged}
 	    onFocus={() => setSuggestionsVsbl (true)}
 	    onBlur={() => setSuggestionsVsbl (false)}
 	    onKeyDown={handleKeyDown}
-	    className="w-full border p-2 rounded"/>
+	    className={inputClass (invalid)}/>
 	<TagSearchBox
 	    suggestions={
 		suggestionsVsbl && suggestions.length > 0 ? suggestions : [] as Tag[]}
@@ -99,4 +105,4 @@ const TagInput: FC<Props> = ({ value, setValue }) => {
       </div>)
 }
 
-export default TagInput
\ No newline at end of file
+export default TagInput
diff --git a/frontend/src/components/common/TextArea.tsx b/frontend/src/components/common/TextArea.tsx
index 64178da..dbcdb18 100644
--- a/frontend/src/components/common/TextArea.tsx
+++ b/frontend/src/components/common/TextArea.tsx
@@ -1,9 +1,25 @@
 import { forwardRef } from 'react'
 
+import { cn } from '@/lib/utils'
+
 import type { TextareaHTMLAttributes } from 'react'
 
-type Props = TextareaHTMLAttributes<HTMLTextAreaElement>
+type Props = TextareaHTMLAttributes<HTMLTextAreaElement> & { invalid?: boolean }
 
 
-export default forwardRef<HTMLTextAreaElement, Props> (({ ...props }, ref) => (
-    <textarea ref={ref} className="rounded border w-full p-2 h-32" {...props}/>))
+export default forwardRef<HTMLTextAreaElement, Props> (
+    ({ className, invalid = false, ...props }, ref) => (
+	<textarea
+	    ref={ref}
+	    aria-invalid={invalid}
+	    className={cn ('rounded border w-full p-2 h-32',
+			   (invalid
+			    ? ['border-red-500 bg-red-50 text-red-900',
+			       'focus:border-red-500 focus:outline-none focus:ring-2',
+			       'focus:ring-red-200',
+			       'dark:border-red-500 dark:bg-red-950/30 dark:text-red-100']
+			    : ['border-gray-300',
+			       'focus:border-blue-500 focus:outline-none focus:ring-2',
+			       'focus:ring-blue-200']),
+			   className)}
+	    {...props}/>))
diff --git a/frontend/src/lib/apiErrors.test.ts b/frontend/src/lib/apiErrors.test.ts
new file mode 100644
index 0000000..fbbc67a
--- /dev/null
+++ b/frontend/src/lib/apiErrors.test.ts
@@ -0,0 +1,79 @@
+import { describe, expect, it, vi } from 'vitest'
+
+const api = vi.hoisted (() => ({
+  isApiError: vi.fn (),
+}))
+
+vi.mock ('@/lib/api', () => api)
+
+describe ('extractValidationError', () => {
+  it ('extracts field and base errors from 422 validation responses', async () => {
+    api.isApiError.mockReturnValueOnce (true)
+
+    const { extractValidationError } = await import ('@/lib/apiErrors')
+    const validationError = extractValidationError<'name'> ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      { name: ['名前は必須です．'] },
+          base_errors: ['全体エラー'],
+        },
+      },
+    })
+
+    expect (validationError).toEqual ({
+      message:     '入力内容を確認してください．',
+      fieldErrors: { name: ['名前は必須です．'] },
+      baseErrors:  ['全体エラー'],
+    })
+  })
+
+  it ('preserves dotted field keys for indexed form rows', async () => {
+    api.isApiError.mockReturnValueOnce (true)
+
+    const { extractValidationError } = await import ('@/lib/apiErrors')
+    const validationError = extractValidationError<'deerjikists.0.platform'> ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          errors:      { 'deerjikists.0.platform': ['プラットフォームを入力してください．'] },
+          base_errors: [],
+        },
+      },
+    })
+
+    expect (validationError?.fieldErrors).toEqual ({
+      'deerjikists0Platform': ['プラットフォームを入力してください．'],
+    })
+  })
+
+  it ('does not treat 400 bad requests as form validation errors', async () => {
+    api.isApiError.mockReturnValueOnce (true)
+
+    const { extractValidationError } = await import ('@/lib/apiErrors')
+    const validationError = extractValidationError ({
+      response: {
+        status: 400,
+        data:   {
+          type:        'bad_request',
+          message:     'リクエストが不正です．',
+          errors:      {},
+          base_errors: ['リクエストが不正です．'],
+        },
+      },
+    })
+
+    expect (validationError).toBeNull ()
+  })
+
+  it ('ignores non-api errors', async () => {
+    api.isApiError.mockReturnValueOnce (false)
+
+    const { extractValidationError } = await import ('@/lib/apiErrors')
+
+    expect (extractValidationError (new Error ('network'))).toBeNull ()
+  })
+})
diff --git a/frontend/src/lib/apiErrors.ts b/frontend/src/lib/apiErrors.ts
new file mode 100644
index 0000000..9a1181f
--- /dev/null
+++ b/frontend/src/lib/apiErrors.ts
@@ -0,0 +1,36 @@
+import toCamel from 'camelcase-keys'
+
+import { isApiError } from '@/lib/api'
+
+export type FieldErrors<T extends string = string> = Partial<Record<T, string[]>>
+
+export type ValidationError<T extends string = string> =
+    { message:      string
+      fieldErrors:  FieldErrors<T>
+      baseErrors:   string[] }
+
+type RawValidationError = { type?:        string
+			    message?:     string
+			    errors?:      Record<string, string[]>
+			    baseErrors?:  string[] }
+
+
+export const extractValidationError = <T extends string = string> (err: unknown) => {
+  if (!(isApiError (err)) || err.response?.status !== 422)
+    return null
+
+  const rawData = toCamel ((err.response.data ?? { }) as Record<string, unknown>,
+			   { deep: true }) as RawValidationError
+  const data: RawValidationError = {
+      type:       rawData.type as string | undefined,
+      message:    rawData.message as string | undefined,
+      errors:     rawData.errors as Record<string, string[]> | undefined,
+      baseErrors: rawData.baseErrors as string[] | undefined }
+
+  if (data.type !== 'validation_error' && !(data.errors))
+    return null
+
+  return { message: data.message ?? '入力内容を確認してください．',
+	   fieldErrors: (data.errors ?? { }) as FieldErrors<T>,
+	   baseErrors: data.baseErrors ?? [] }
+}
diff --git a/frontend/src/lib/prefetchers.test.ts b/frontend/src/lib/prefetchers.test.ts
index b33cb65..8a341a3 100644
--- a/frontend/src/lib/prefetchers.test.ts
+++ b/frontend/src/lib/prefetchers.test.ts
@@ -10,6 +10,7 @@ const postsApi = vi.hoisted (() => ({
 }))
 
 const tagsApi = vi.hoisted (() => ({
+  fetchNicoTags:   vi.fn (),
   fetchTag:        vi.fn (),
   fetchTagByName:  vi.fn (),
   fetchTagChanges: vi.fn (),
@@ -37,6 +38,7 @@ describe ('prefetchForURL', () => {
     postsApi.fetchPost.mockResolvedValue ({ id: 1 })
     postsApi.fetchPostChanges.mockResolvedValue ({ versions: [], count: 0 })
     tagsApi.fetchTags.mockResolvedValue ({ tags: [], count: 0 })
+    tagsApi.fetchNicoTags.mockResolvedValue ({ tags: [], count: 0 })
     tagsApi.fetchTag.mockResolvedValue ({ id: 1 })
     tagsApi.fetchTagByName.mockResolvedValue (null)
     tagsApi.fetchTagChanges.mockResolvedValue ({ versions: [], count: 0 })
@@ -85,6 +87,32 @@ describe ('prefetchForURL', () => {
     )
   })
 
+  it ('prefetches nico tag indexes and their alias from query parameters', async () => {
+    await prefetchForURL (
+        qc (),
+        'http://localhost/tags/nico?name=source&linked_tag=destination'
+	    + '&link_status=linked&page=3&limit=10',
+    )
+    await prefetchForURL (qc (), 'http://localhost/nico/tags?page=2')
+
+    expect (tagsApi.fetchNicoTags).toHaveBeenNthCalledWith (1, {
+      name:       'source',
+      linkedTag:  'destination',
+      linkStatus: 'linked',
+      page:       3,
+      limit:      10,
+      order:      'updated_at:desc',
+    })
+    expect (tagsApi.fetchNicoTags).toHaveBeenNthCalledWith (2, {
+      name:       '',
+      linkedTag:  '',
+      linkStatus: 'all',
+      page:       2,
+      limit:      20,
+      order:      'updated_at:desc',
+    })
+  })
+
   it ('prefetches wiki show pages and related tag/post data', async () => {
     wikiApi.fetchWikiPageByTitle.mockResolvedValueOnce ({
       id:    3,
diff --git a/frontend/src/lib/prefetchers.ts b/frontend/src/lib/prefetchers.ts
index 5dc9d70..38f51cc 100644
--- a/frontend/src/lib/prefetchers.ts
+++ b/frontend/src/lib/prefetchers.ts
@@ -3,7 +3,7 @@ import { match } from 'path-to-regexp'
 
 import { fetchPost, fetchPosts, fetchPostChanges } from '@/lib/posts'
 import { postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
-import { fetchTagByName, fetchTag, fetchTagChanges, fetchTags } from '@/lib/tags'
+import { fetchNicoTags, fetchTagByName, fetchTag, fetchTagChanges, fetchTags } from '@/lib/tags'
 import { fetchWikiPage,
 	 fetchWikiPageByTitle,
 	 fetchWikiPages } from '@/lib/wiki'
@@ -170,6 +170,24 @@ const prefetchTagsIndex: Prefetcher = async (qc, url) => {
 }
 
 
+const prefetchNicoTagsIndex: Prefetcher = async (qc, url) => {
+  const keys = {
+      name:       url.searchParams.get ('name') ?? '',
+      linkedTag:  url.searchParams.get ('linked_tag') ?? '',
+      linkStatus: (url.searchParams.get ('link_status') || 'all') as
+		  'all' | 'linked' | 'unlinked',
+      page:       Number (url.searchParams.get ('page') || 1),
+      limit:      Number (url.searchParams.get ('limit') || 20),
+      order:      (url.searchParams.get ('order') || 'updated_at:desc') as
+		  'name:asc' | 'name:desc' | 'created_at:asc' | 'created_at:desc'
+		  | 'updated_at:asc' | 'updated_at:desc' }
+
+  await qc.prefetchQuery ({
+      queryKey: tagsKeys.nicoIndex (keys),
+      queryFn:  () => fetchNicoTags (keys) })
+}
+
+
 const prefetchTagShow: Prefetcher = async (qc, url) => {
   const m = mTag (url.pathname)
   if (!(m))
@@ -206,6 +224,8 @@ export const routePrefetchers: { test: (u: URL) => boolean; run: Prefetcher }[]
 		  && Boolean (mWiki (u.pathname))),
       run: prefetchWikiPageShow },
     { test: u => u.pathname === '/tags', run: prefetchTagsIndex },
+    { test: u => ['/tags/nico', '/nico/tags'].includes (u.pathname),
+      run: prefetchNicoTagsIndex },
     { test: u => (!(['/tags/nico', '/tags/changes'].includes (u.pathname))
 		  && Boolean (mTag (u.pathname))),
       run: prefetchTagShow },
diff --git a/frontend/src/lib/queryKeys.ts b/frontend/src/lib/queryKeys.ts
index 6ac3f21..b7dff2b 100644
--- a/frontend/src/lib/queryKeys.ts
+++ b/frontend/src/lib/queryKeys.ts
@@ -1,4 +1,4 @@
-import type { FetchPostsParams, FetchTagsParams } from '@/types'
+import type { FetchNicoTagsParams, FetchPostsParams, FetchTagsParams } from '@/types'
 
 export const postsKeys = {
     root:     ['posts'] as const,
@@ -11,6 +11,8 @@ export const postsKeys = {
 export const tagsKeys = {
     root:         ['tags'] as const,
     index:        (p: FetchTagsParams) => ['tags', 'index', p] as const,
+    nicoRoot:     ['tags', 'nico'] as const,
+    nicoIndex:    (p: FetchNicoTagsParams) => ['tags', 'nico', 'index', p] as const,
     show:         (name: string) => ['tags', name] as const,
     changes:      (p: { id?: string; page: number; limit: number }) =>
 		      ['tags', 'changes', p] as const,
diff --git a/frontend/src/lib/tags.ts b/frontend/src/lib/tags.ts
index 74eba45..9ac8788 100644
--- a/frontend/src/lib/tags.ts
+++ b/frontend/src/lib/tags.ts
@@ -1,6 +1,11 @@
 import { apiGet } from '@/lib/api'
 
-import type { Deerjikist, FetchTagsParams, Tag, TagVersion } from '@/types'
+import type { Deerjikist,
+	      FetchNicoTagsParams,
+	      FetchTagsParams,
+	      NicoTag,
+	      Tag,
+	      TagVersion } from '@/types'
 
 
 export const fetchTags = async (
@@ -23,6 +28,19 @@ export const fetchTags = async (
 	...(order && { order }) } })
 
 
+export const fetchNicoTags = async (
+    { name, linkedTag, linkStatus, page, limit, order }: FetchNicoTagsParams,
+): Promise<{ tags:  NicoTag[]
+	     count: number }> =>
+    await apiGet ('/tags/nico', { params: {
+	page,
+	limit,
+	name,
+	linked_tag:  linkedTag,
+	link_status: linkStatus === 'all' ? '' : linkStatus,
+	order } })
+
+
 export const fetchTag = async (id: string): Promise<Tag | null> => {
   try
   {
diff --git a/frontend/src/lib/useValidationErrors.ts b/frontend/src/lib/useValidationErrors.ts
new file mode 100644
index 0000000..da0ebc6
--- /dev/null
+++ b/frontend/src/lib/useValidationErrors.ts
@@ -0,0 +1,28 @@
+import { useState } from 'react'
+
+import { extractValidationError } from '@/lib/apiErrors'
+
+import type { FieldErrors } from '@/lib/apiErrors'
+
+
+export const useValidationErrors = <T extends string> () => {
+  const [baseErrors, setBaseErrors] = useState<string[]> ([])
+  const [fieldErrors, setFieldErrors] = useState<FieldErrors<T>> ({ })
+
+  const clearValidationErrors = () => {
+    setBaseErrors ([])
+    setFieldErrors ({ })
+  }
+
+  const applyValidationError = (error: unknown): boolean => {
+    const validationError = extractValidationError<T> (error)
+    if (!(validationError))
+      return false
+
+    setBaseErrors (validationError.baseErrors)
+    setFieldErrors (validationError.fieldErrors)
+    return true
+  }
+
+  return { baseErrors, fieldErrors, clearValidationErrors, applyValidationError }
+}
diff --git a/frontend/src/lib/users.test.ts b/frontend/src/lib/users.test.ts
new file mode 100644
index 0000000..aed765b
--- /dev/null
+++ b/frontend/src/lib/users.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest'
+
+import { canEditContent } from '@/lib/users'
+
+import type { UserRole } from '@/types'
+
+const userWithRole = (role: UserRole) => ({ role })
+
+describe ('user permission helpers', () => {
+  it ('allows admins and members to edit content', () => {
+    expect (canEditContent (userWithRole ('admin'))).toBe (true)
+    expect (canEditContent (userWithRole ('member'))).toBe (true)
+  })
+
+  it ('does not allow guests or missing users to edit content', () => {
+    expect (canEditContent (userWithRole ('guest'))).toBe (false)
+    expect (canEditContent (null)).toBe (false)
+    expect (canEditContent (undefined)).toBe (false)
+  })
+})
diff --git a/frontend/src/lib/users.ts b/frontend/src/lib/users.ts
new file mode 100644
index 0000000..212e66e
--- /dev/null
+++ b/frontend/src/lib/users.ts
@@ -0,0 +1,7 @@
+import type { User, UserRole } from '@/types'
+
+const CONTENT_EDITOR_ROLES: readonly UserRole[] = ['admin', 'member']
+
+export const canEditContent = (
+    user: Pick<User, 'role'> | null | undefined,
+): boolean => user != null && CONTENT_EDITOR_ROLES.includes (user.role)
diff --git a/frontend/src/lib/utils.ts b/frontend/src/lib/utils.ts
index aac3785..69ccbb8 100644
--- a/frontend/src/lib/utils.ts
+++ b/frontend/src/lib/utils.ts
@@ -71,3 +71,15 @@ export const originalCreatedAtString = (
 	       .join (' '))
   return rtn === '〜' ? '年月日不詳' : rtn
 }
+
+
+export const inputClass = (invalid?: boolean, className?: string): string =>
+    cn ('w-full rounded border p-2',
+	(invalid
+	 ? ['border-red-500 bg-red-50 text-red-900',
+	    'placeholder:text-red-300',
+	    'focus:border-red-500 focus:outline-none focus:ring-2 focus:ring-red-200',
+	    'dark:border-red-500 dark:bg-red-950/30 dark:text-red-100']
+	 : ['border-gray-300',
+	    'focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200']),
+	className)
diff --git a/frontend/src/pages/deerjikists/DeerjikistDetailPage.test.tsx b/frontend/src/pages/deerjikists/DeerjikistDetailPage.test.tsx
new file mode 100644
index 0000000..ae60500
--- /dev/null
+++ b/frontend/src/pages/deerjikists/DeerjikistDetailPage.test.tsx
@@ -0,0 +1,72 @@
+import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { Route, Routes } from 'react-router-dom'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import DeerjikistDetailPage from '@/pages/deerjikists/DeerjikistDetailPage'
+import { buildTag } from '@/test/factories'
+import { renderWithProviders } from '@/test/render'
+
+const tagsApi = vi.hoisted (() => ({
+  fetchDeerjikistsByTag: vi.fn (),
+}))
+
+const api = vi.hoisted (() => ({
+  apiPut:     vi.fn (),
+  isApiError: vi.fn (),
+}))
+
+const toastApi = vi.hoisted (() => ({
+  toast: vi.fn (),
+}))
+
+vi.mock ('@/lib/tags', () => tagsApi)
+vi.mock ('@/lib/api', () => api)
+vi.mock ('@/components/ui/use-toast', () => toastApi)
+
+const renderPage = () =>
+  renderWithProviders (
+      <Routes>
+        <Route path="/tags/:id/deerjikists" element={<DeerjikistDetailPage/>}/>
+      </Routes>,
+      { route: '/tags/7/deerjikists' },
+  )
+
+describe ('DeerjikistDetailPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.isApiError.mockReturnValue (false)
+  })
+
+  it ('shows indexed validation errors returned for deerjikist rows', async () => {
+    tagsApi.fetchDeerjikistsByTag.mockResolvedValueOnce ({
+      tag:         buildTag ({ id: 7, name: 'deerjika', category: 'deerjikist' }),
+      deerjikists: [{ platform: null, code: 'abc' }],
+    })
+    api.isApiError.mockReturnValue (true)
+    api.apiPut.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      { 'deerjikists.0.platform': ['プラットフォームを入力してください．'] },
+          base_errors: [],
+        },
+      },
+    })
+
+    renderPage ()
+
+    await screen.findByDisplayValue ('abc')
+    fireEvent.submit (screen.getByRole ('button', { name: '更新' }).closest ('form')!)
+
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledWith (
+          '/tags/7/deerjikists',
+          [{ platform: null, code: 'abc' }],
+      )
+    })
+    expect (await screen.findByText ('プラットフォームを入力してください．')).toBeInTheDocument ()
+    expect (screen.getByRole ('combobox')).toHaveAttribute ('aria-invalid', 'true')
+  })
+})
diff --git a/frontend/src/pages/deerjikists/DeerjikistDetailPage.tsx b/frontend/src/pages/deerjikists/DeerjikistDetailPage.tsx
index 69444b1..99dd7ad 100644
--- a/frontend/src/pages/deerjikists/DeerjikistDetailPage.tsx
+++ b/frontend/src/pages/deerjikists/DeerjikistDetailPage.tsx
@@ -3,7 +3,8 @@ import { useEffect, useMemo, useState } from 'react'
 import { useParams } from 'react-router-dom'
 
 import TagLink from '@/components/TagLink'
-import Label from '@/components/common/Label'
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
@@ -11,12 +12,16 @@ import { PLATFORM_NAMES, PLATFORMS } from '@/consts'
 import { apiPut } from '@/lib/api'
 import { tagsKeys } from '@/lib/queryKeys'
 import { fetchDeerjikistsByTag } from '@/lib/tags'
-import { cn } from '@/lib/utils'
+import { cn, inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC, FormEvent } from 'react'
 
 import type { Deerjikist, Platform } from '@/types'
 
+type DeerjikistFormField =
+    'deerjikists' | `deerjikists${ number }Platform` | `deerjikists${ number }Code`
+
 
 const DeerjikistDetailPage: FC = () => {
   const { id } = useParams ()
@@ -31,11 +36,14 @@ const DeerjikistDetailPage: FC = () => {
   const [data, setData] =
       useState<(Omit<Deerjikist, 'platform'> & { platform: Platform | null })[]> ([])
   const [disabled, setDisabled] = useState (true)
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<DeerjikistFormField> ()
 
   const qc = useQueryClient ()
 
   const handleSubmit = async (e: FormEvent) => {
     e.preventDefault ()
+    clearValidationErrors ()
 
     try
     {
@@ -46,8 +54,9 @@ const DeerjikistDetailPage: FC = () => {
 
       toast ({ description: '更新しました．' })
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '更新失敗', description: '入力内容を確認してください．' })
     }
     finally
@@ -76,6 +85,9 @@ const DeerjikistDetailPage: FC = () => {
 	      </PageTitle>
 
 	      <form onSubmit={handleSubmit} className="my-4 space-y-2">
+		<FieldError messages={baseErrors}/>
+		<FieldError messages={fieldErrors.deerjikists}/>
+
 		{data.map ((datum, i) => (
 		    <fieldset key={i} className="min-w-0 rounded-lg border border-gray-300
 						 dark:border-gray-700 p-4">
@@ -91,40 +103,48 @@ const DeerjikistDetailPage: FC = () => {
 		      </legend>
 
 		      {/* プラットフォーム */}
-		      <div>
-			<Label>プラットフォーム</Label>
-			<select
-			    className="w-full border p-2 rounded"
-			    disabled={disabled}
-			    value={datum.platform ?? ''}
-			    onChange={e => setData (prev => {
-			      const rtn = [...prev]
-			      rtn[i] = { ...rtn[i],
-					 platform: (e.target.value || null) as Platform | null }
-			      return rtn
-			    })}>
-			  <option value="">&nbsp;</option>
-			  {PLATFORMS.map (p => (
-			      <option key={p} value={p}>
-				{PLATFORM_NAMES[p]}
-			      </option>))}
-			</select>
-		      </div>
+		      <FormField
+			  label="プラットフォーム"
+			  messages={fieldErrors[`deerjikists${ i }Platform`]}>
+			{({ describedBy, invalid }) => (
+			    <select
+				disabled={disabled}
+				value={datum.platform ?? ''}
+				aria-describedby={describedBy}
+				aria-invalid={invalid}
+				className={inputClass (invalid)}
+				onChange={e => setData (prev => {
+				  const rtn = [...prev]
+				  rtn[i] = { ...rtn[i],
+					     platform: (e.target.value || null) as Platform | null }
+				  return rtn
+				})}>
+			      <option value="">&nbsp;</option>
+			      {PLATFORMS.map (p => (
+				  <option key={p} value={p}>
+				    {PLATFORM_NAMES[p]}
+				  </option>))}
+			    </select>)}
+		      </FormField>
 
 		      {/* コード */}
-		      <div>
-			<Label>コード</Label>
-			<input
-			    type="text"
-			    disabled={disabled}
-			    className="w-full border p-2 rounded"
-			    value={datum.code}
-			    onChange={e => setData (prev => {
-			      const rtn = [...prev]
-			      rtn[i] = { ...rtn[i], code: e.target.value }
-			      return rtn
-			    })}/>
-		      </div>
+		      <FormField
+			  label="コード"
+			  messages={fieldErrors[`deerjikists${ i }Code`]}>
+			{({ describedBy, invalid }) => (
+			    <input
+				type="text"
+				disabled={disabled}
+				value={datum.code}
+				aria-describedby={describedBy}
+				aria-invalid={invalid}
+				className={inputClass (invalid)}
+				onChange={e => setData (prev => {
+				  const rtn = [...prev]
+				  rtn[i] = { ...rtn[i], code: e.target.value }
+				  return rtn
+				})}/>)}
+		      </FormField>
 		    </fieldset>
 		))}
 
diff --git a/frontend/src/pages/materials/MaterialDetailPage.tsx b/frontend/src/pages/materials/MaterialDetailPage.tsx
index 55e242b..09f537e 100644
--- a/frontend/src/pages/materials/MaterialDetailPage.tsx
+++ b/frontend/src/pages/materials/MaterialDetailPage.tsx
@@ -4,7 +4,8 @@ import { useParams } from 'react-router-dom'
 
 import TagLink from '@/components/TagLink'
 import WikiBody from '@/components/WikiBody'
-import Label from '@/components/common/Label'
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import TabGroup, { Tab } from '@/components/common/TabGroup'
 import TagInput from '@/components/common/TagInput'
@@ -13,6 +14,8 @@ import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiGet, apiPut } from '@/lib/api'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC } from 'react'
 
@@ -20,6 +23,8 @@ import type { Material, Tag } from '@/types'
 
 type MaterialWithTag = Material & { tag: Tag }
 
+type MaterialFormField = 'tag' | 'file' | 'url'
+
 
 const MaterialDetailPage: FC = () => {
   const { id } = useParams ()
@@ -31,8 +36,12 @@ const MaterialDetailPage: FC = () => {
   const [sending, setSending] = useState (false)
   const [tag, setTag] = useState ('')
   const [url, setURL] = useState ('')
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<MaterialFormField> ()
 
   const handleSubmit = async () => {
+    clearValidationErrors ()
+
     const formData = new FormData
     if (tag.trim ())
       formData.append ('tag', tag)
@@ -48,8 +57,9 @@ const MaterialDetailPage: FC = () => {
       setMaterial (data)
       toast ({ title: '更新成功!' })
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '更新失敗……', description: '入力を見直してください．' })
     }
     finally
@@ -118,54 +128,66 @@ const MaterialDetailPage: FC = () => {
 
 		<Tab name="編輯">
 		  <div className="max-w-wl pt-2 space-y-4">
+		    <FieldError messages={baseErrors}/>
+
 		    {/* タグ */}
-		    <div>
-		      <Label>タグ</Label>
-		      <TagInput value={tag} setValue={setTag}/>
-		    </div>
+		    <FormField label="タグ" messages={fieldErrors.tag}>
+		      {({ describedBy, invalid }) => (
+			  <TagInput
+			      describedBy={describedBy}
+			      invalid={invalid}
+			      value={tag}
+			      setValue={setTag}/>)}
+		    </FormField>
 
 		    {/* ファイル */}
-		    <div>
-		      <Label>ファイル</Label>
-		      <input
-			  type="file"
-			  accept="image/*,video/*,audio/*"
-			  onChange={e => {
-			    const f = e.target.files?.[0]
-			    setFile (f ?? null)
-			    setFilePreview (f ? URL.createObjectURL (f) : '')
-			  }}/>
-		      {(file && filePreview) && (
-			  (/image\/.*/.test (file.type) && (
-			      <img
-				  src={filePreview}
-				  alt="preview"
-				  className="mt-2 max-h-48 rounded border"/>))
-			  || (/video\/.*/.test (file.type) && (
-			      <video
-				  src={filePreview}
-				  controls
-				  className="mt-2 max-h-48 rounded border"/>))
-			  || (/audio\/.*/.test (file.type) && (
-			      <audio
-				  src={filePreview}
-				  controls
-				  className="mt-2 max-h-48"/>))
-			  || (
-			      <p className="text-red-600 dark:text-red-400">
-				その形式のファイルには対応していません．
-			      </p>))}
-		    </div>
+		    <FormField label="ファイル" messages={fieldErrors.file}>
+		      {({ describedBy, invalid }) => (
+			  <>
+			    <input
+				type="file"
+				accept="image/*,video/*,audio/*"
+				aria-describedby={describedBy}
+				aria-invalid={invalid}
+				onChange={e => {
+				  const f = e.target.files?.[0]
+				  setFile (f ?? null)
+				  setFilePreview (f ? URL.createObjectURL (f) : '')
+				}}/>
+			    {(file && filePreview) && (
+				(/image\/.*/.test (file.type) && (
+				    <img
+					src={filePreview}
+					alt="preview"
+					className="mt-2 max-h-48 rounded border"/>))
+				|| (/video\/.*/.test (file.type) && (
+				    <video
+					src={filePreview}
+					controls
+					className="mt-2 max-h-48 rounded border"/>))
+				|| (/audio\/.*/.test (file.type) && (
+				    <audio
+					src={filePreview}
+					controls
+					className="mt-2 max-h-48"/>))
+				|| (
+				    <p className="text-red-600 dark:text-red-400">
+				      その形式のファイルには対応していません．
+				    </p>))}
+			  </>)}
+		    </FormField>
 
 		    {/* 参考 URL */}
-		    <div>
-		      <Label>参考 URL</Label>
-		      <input
-			  type="url"
-			  value={url}
-			  onChange={e => setURL (e.target.value)}
-			  className="w-full border p-2 rounded"/>
-		    </div>
+		    <FormField label="参考 URL" messages={fieldErrors.url}>
+		      {({ describedBy, invalid }) => (
+			  <input
+			      type="url"
+			      value={url}
+			      onChange={e => setURL (e.target.value)}
+			      aria-describedby={describedBy}
+			      aria-invalid={invalid}
+			      className={inputClass (invalid)}/>)}
+		    </FormField>
 
 		    {/* 送信 */}
 		    <Button
@@ -181,4 +203,4 @@ const MaterialDetailPage: FC = () => {
       </MainArea>)
 }
 
-export default MaterialDetailPage
\ No newline at end of file
+export default MaterialDetailPage
diff --git a/frontend/src/pages/materials/MaterialNewPage.test.tsx b/frontend/src/pages/materials/MaterialNewPage.test.tsx
index c33a5dc..c67e55b 100644
--- a/frontend/src/pages/materials/MaterialNewPage.test.tsx
+++ b/frontend/src/pages/materials/MaterialNewPage.test.tsx
@@ -1,11 +1,13 @@
 import { fireEvent, screen, waitFor } from '@testing-library/react'
-import { describe, expect, it, vi } from 'vitest'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import MaterialNewPage from '@/pages/materials/MaterialNewPage'
 import { renderWithProviders } from '@/test/render'
 
 const api = vi.hoisted (() => ({
-  apiPost: vi.fn (),
+  apiGet:     vi.fn (),
+  apiPost:    vi.fn (),
+  isApiError: vi.fn (),
 }))
 
 const toastApi = vi.hoisted (() => ({
@@ -16,6 +18,12 @@ vi.mock ('@/lib/api', () => api)
 vi.mock ('@/components/ui/use-toast', () => toastApi)
 
 describe ('MaterialNewPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.apiGet.mockResolvedValue ([])
+    api.isApiError.mockReturnValue (false)
+  })
+
   it ('initializes tag from query and submits form data', async () => {
     api.apiPost.mockResolvedValueOnce ({})
 
@@ -35,4 +43,30 @@ describe ('MaterialNewPage', () => {
     expect (formData.get ('url')).toBe ('https://example.com/ref')
     expect (toastApi.toast).toHaveBeenCalledWith ({ title: '送信成功!' })
   })
+
+  it ('shows validation errors for file and url fields', async () => {
+    api.isApiError.mockReturnValue (true)
+    api.apiPost.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      {
+            file: ['ファイルまたは URL は必須です．'],
+            url:  ['ファイルまたは URL は必須です．'],
+          },
+          base_errors: [],
+        },
+      },
+    })
+
+    renderWithProviders (<MaterialNewPage/>)
+
+    fireEvent.change (screen.getAllByRole ('textbox')[0], { target: { value: '虹夏' } })
+    fireEvent.click (screen.getByRole ('button', { name: '追加' }))
+
+    expect (await screen.findAllByText ('ファイルまたは URL は必須です．')).toHaveLength (2)
+    expect (screen.getAllByRole ('textbox')[1]).toHaveAttribute ('aria-invalid', 'true')
+  })
 })
diff --git a/frontend/src/pages/materials/MaterialNewPage.tsx b/frontend/src/pages/materials/MaterialNewPage.tsx
index f2a1832..bfbb357 100644
--- a/frontend/src/pages/materials/MaterialNewPage.tsx
+++ b/frontend/src/pages/materials/MaterialNewPage.tsx
@@ -2,8 +2,9 @@ import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useLocation, useNavigate } from 'react-router-dom'
 
+import FieldError from '@/components/common/FieldError'
 import Form from '@/components/common/Form'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import TagInput from '@/components/common/TagInput'
 import MainArea from '@/components/layout/MainArea'
@@ -11,9 +12,13 @@ import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiPost } from '@/lib/api'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC } from 'react'
 
+type MaterialFormField = 'tag' | 'file' | 'url'
+
 
 const MaterialNewPage: FC = () => {
   const location = useLocation ()
@@ -27,8 +32,12 @@ const MaterialNewPage: FC = () => {
   const [sending, setSending] = useState (false)
   const [tag, setTag] = useState (tagQuery)
   const [url, setURL] = useState ('')
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<MaterialFormField> ()
 
   const handleSubmit = async () => {
+    clearValidationErrors ()
+
     const formData = new FormData
     if (tag)
       formData.append ('tag', tag)
@@ -44,8 +53,9 @@ const MaterialNewPage: FC = () => {
       toast ({ title: '送信成功!' })
       navigate (`/materials?tag=${ encodeURIComponent (tag) }`)
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '送信失敗……', description: '入力を見直してください．' })
     }
     finally
@@ -62,55 +72,66 @@ const MaterialNewPage: FC = () => {
 
 	<Form>
 	  <PageTitle>素材追加</PageTitle>
+	  <FieldError messages={baseErrors}/>
 
 	  {/* タグ */}
-	  <div>
-	    <Label>タグ</Label>
-	    <TagInput value={tag} setValue={setTag}/>
-	  </div>
+	  <FormField label="タグ" messages={fieldErrors.tag}>
+	    {({ describedBy, invalid }) => (
+		<TagInput
+		    describedBy={describedBy}
+		    invalid={invalid}
+		    value={tag}
+		    setValue={setTag}/>)}
+	  </FormField>
 
 	  {/* ファイル */}
-	  <div>
-	    <Label>ファイル</Label>
-	    <input
-		type="file"
-		accept="image/*,video/*,audio/*"
-		onChange={e => {
-		  const f = e.target.files?.[0]
-		  setFile (f ?? null)
-		  setFilePreview (f ? URL.createObjectURL (f) : '')
-		}}/>
-	    {(file && filePreview) && (
-		(/image\/.*/.test (file.type) && (
-		    <img
-			src={filePreview}
-			alt="preview"
-			className="mt-2 max-h-48 rounded border"/>))
-		|| (/video\/.*/.test (file.type) && (
-		    <video
-			src={filePreview}
-			controls
-			className="mt-2 max-h-48 rounded border"/>))
-		|| (/audio\/.*/.test (file.type) && (
-		    <audio
-			src={filePreview}
-			controls
-			className="mt-2 max-h-48"/>))
-		|| (
-		    <p className="text-red-600 dark:text-red-400">
-		      その形式のファイルには対応していません．
-		    </p>))}
-	  </div>
+	  <FormField label="ファイル" messages={fieldErrors.file}>
+	    {({ describedBy, invalid }) => (
+		<>
+		  <input
+		      type="file"
+		      accept="image/*,video/*,audio/*"
+		      aria-describedby={describedBy}
+		      aria-invalid={invalid}
+		      onChange={e => {
+			const f = e.target.files?.[0]
+			setFile (f ?? null)
+			setFilePreview (f ? URL.createObjectURL (f) : '')
+		      }}/>
+		  {(file && filePreview) && (
+		      (/image\/.*/.test (file.type) && (
+			  <img
+			      src={filePreview}
+			      alt="preview"
+			      className="mt-2 max-h-48 rounded border"/>))
+		      || (/video\/.*/.test (file.type) && (
+			  <video
+			      src={filePreview}
+			      controls
+			      className="mt-2 max-h-48 rounded border"/>))
+		      || (/audio\/.*/.test (file.type) && (
+			  <audio
+			      src={filePreview}
+			      controls
+			      className="mt-2 max-h-48"/>))
+		      || (
+			  <p className="text-red-600 dark:text-red-400">
+			    その形式のファイルには対応していません．
+			  </p>))}
+		</>)}
+	  </FormField>
 
 	  {/* 参考 URL */}
-	  <div>
-	    <Label>参考 URL</Label>
-	    <input
-		type="url"
-		value={url}
-		onChange={e => setURL (e.target.value)}
-		className="w-full border p-2 rounded"/>
-	  </div>
+	  <FormField label="参考 URL" messages={fieldErrors.url}>
+	    {({ describedBy, invalid }) => (
+		<input
+		    type="url"
+		    value={url}
+		    onChange={e => setURL (e.target.value)}
+		    aria-describedby={describedBy}
+		    aria-invalid={invalid}
+		    className={inputClass (invalid)}/>)}
+	  </FormField>
 
 	  {/* 送信 */}
 	  <Button
@@ -123,4 +144,4 @@ const MaterialNewPage: FC = () => {
       </MainArea>)
 }
 
-export default MaterialNewPage
\ No newline at end of file
+export default MaterialNewPage
diff --git a/frontend/src/pages/materials/MaterialSearchPage.tsx b/frontend/src/pages/materials/MaterialSearchPage.tsx
index f8f3634..ca5888a 100644
--- a/frontend/src/pages/materials/MaterialSearchPage.tsx
+++ b/frontend/src/pages/materials/MaterialSearchPage.tsx
@@ -1,7 +1,7 @@
 import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import TagInput from '@/components/common/TagInput'
 import MainArea from '@/components/layout/MainArea'
@@ -29,23 +29,23 @@ const MaterialSearchPage: FC = () => {
 
 	  <form onSubmit={handleSearch} className="space-y-2">
 	    {/* タグ */}
-	    <div>
-	      <Label>タグ</Label>
-	      <TagInput
-		  value={tagName}
-		  setValue={setTagName}/>
-	    </div>
+	    <FormField label="タグ">
+	      {() => (
+		  <TagInput
+		      value={tagName}
+		      setValue={setTagName}/>)}
+	    </FormField>
 
 	    {/* 親タグ */}
-	    <div>
-	      <Label>親タグ</Label>
-	      <TagInput
-		  value={parentTagName}
-		  setValue={setParentTagName}/>
-	    </div>
+	    <FormField label="親タグ">
+	      {() => (
+		  <TagInput
+		      value={parentTagName}
+		      setValue={setParentTagName}/>)}
+	    </FormField>
 	  </form>
 	</div>
       </MainArea>)
 }
 
-export default MaterialSearchPage
\ No newline at end of file
+export default MaterialSearchPage
diff --git a/frontend/src/pages/posts/PostDetailPage.tsx b/frontend/src/pages/posts/PostDetailPage.tsx
index 906c5cb..7c7f958 100644
--- a/frontend/src/pages/posts/PostDetailPage.tsx
+++ b/frontend/src/pages/posts/PostDetailPage.tsx
@@ -3,7 +3,6 @@ import { motion } from 'framer-motion'
 import { useEffect, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useParams } from 'react-router-dom'
-
 import PostEditForm from '@/components/PostEditForm'
 import PostEmbed from '@/components/PostEmbed'
 import PostList from '@/components/PostList'
@@ -16,6 +15,7 @@ import { SITE_TITLE } from '@/config'
 import { isApiError } from '@/lib/api'
 import { fetchPost, toggleViewedFlg } from '@/lib/posts'
 import { postsKeys, tagsKeys } from '@/lib/queryKeys'
+import { canEditContent } from '@/lib/users'
 import { cn } from '@/lib/utils'
 import NotFound from '@/pages/NotFound'
 import ServiceUnavailable from '@/pages/ServiceUnavailable'
@@ -28,6 +28,7 @@ type Props = { user: User | null }
 
 
 const PostDetailPage: FC<Props> = ({ user }) => {
+  const editable = canEditContent (user)
   const { id } = useParams ()
   const postId = String (id ?? '')
   const postKey = postsKeys.show (postId)
@@ -164,7 +165,7 @@ const PostDetailPage: FC<Props> = ({ user }) => {
 		     ? <PostList posts={post.related}/>
 		     : 'まだないよ（笑）'}
 		  </Tab>
-		  {['admin', 'member'].some (r => user?.role === r) && (
+		  {editable && (
 		      <Tab name="編輯">
 			<PostEditForm
 			    post={post}
diff --git a/frontend/src/pages/posts/PostNewPage.test.tsx b/frontend/src/pages/posts/PostNewPage.test.tsx
index f14b318..b8c909e 100644
--- a/frontend/src/pages/posts/PostNewPage.test.tsx
+++ b/frontend/src/pages/posts/PostNewPage.test.tsx
@@ -1,13 +1,14 @@
 import { fireEvent, screen, waitFor } from '@testing-library/react'
-import { describe, expect, it, vi } from 'vitest'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import PostNewPage from '@/pages/posts/PostNewPage'
 import { buildUser } from '@/test/factories'
 import { renderWithProviders } from '@/test/render'
 
 const api = vi.hoisted (() => ({
-  apiGet:  vi.fn (),
-  apiPost: vi.fn (),
+  apiGet:     vi.fn (),
+  apiPost:    vi.fn (),
+  isApiError: vi.fn (),
 }))
 
 const toastApi = vi.hoisted (() => ({
@@ -18,6 +19,11 @@ vi.mock ('@/lib/api', () => api)
 vi.mock ('@/components/ui/use-toast', () => toastApi)
 
 describe ('PostNewPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.isApiError.mockReturnValue (false)
+  })
+
   it ('blocks guests', () => {
     renderWithProviders (<PostNewPage user={buildUser ({ role: 'guest' })}/>)
 
@@ -55,4 +61,36 @@ describe ('PostNewPage', () => {
     expect (formData.get ('tags')).toBe ('tag1 tag2')
     expect (toastApi.toast).toHaveBeenCalledWith ({ title: '投稿成功!' })
   })
+
+  it ('shows 422 validation errors for post fields', async () => {
+    api.apiGet.mockResolvedValue ([])
+    api.isApiError.mockReturnValue (true)
+    api.apiPost.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      { tags: ['ニコニコ・タグは直接指定できません．'] },
+          base_errors: ['投稿内容を確認してください．'],
+        },
+      },
+    })
+
+    renderWithProviders (<PostNewPage user={buildUser ({ role: 'member' })}/>)
+
+    const checkboxes = screen.getAllByRole ('checkbox', { name: '自動' })
+    fireEvent.click (checkboxes[0])
+    fireEvent.click (checkboxes[1])
+
+    const textboxes = screen.getAllByRole ('textbox')
+    fireEvent.change (textboxes[0], { target: { value: 'https://example.com/post' } })
+    fireEvent.change (textboxes[1], { target: { value: '投稿タイトル' } })
+    fireEvent.change (textboxes[3], { target: { value: 'nico:nico_tag' } })
+    fireEvent.click (screen.getByRole ('button', { name: '追加' }))
+
+    expect (await screen.findByText ('投稿内容を確認してください．')).toBeInTheDocument ()
+    expect (screen.getByText ('ニコニコ・タグは直接指定できません．')).toBeInTheDocument ()
+    expect (screen.getAllByRole ('textbox')[3]).toHaveAttribute ('aria-invalid', 'true')
+  })
 })
diff --git a/frontend/src/pages/posts/PostNewPage.tsx b/frontend/src/pages/posts/PostNewPage.tsx
index d8588d9..f5915db 100644
--- a/frontend/src/pages/posts/PostNewPage.tsx
+++ b/frontend/src/pages/posts/PostNewPage.tsx
@@ -4,14 +4,18 @@ import { useNavigate } from 'react-router-dom'
 
 import PostFormTagsArea from '@/components/PostFormTagsArea'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
+import FieldError from '@/components/common/FieldError'
 import Form from '@/components/common/Form'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiGet, apiPost } from '@/lib/api'
+import { canEditContent } from '@/lib/users'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 import Forbidden from '@/pages/Forbidden'
 
 import type { FC } from 'react'
@@ -20,12 +24,18 @@ import type { User } from '@/types'
 
 type Props = { user: User | null }
 
+type PostFormField =
+    'url' | 'title' | 'tags' | 'parentPostIds' | 'originalCreatedAt' | 'thumbnail'
+
 
 const PostNewPage: FC<Props> = ({ user }) => {
-  const editable = ['admin', 'member'].some (r => user?.role === r)
+  const editable = canEditContent (user)
 
   const navigate = useNavigate ()
 
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<PostFormField> ()
+
   const [originalCreatedBefore, setOriginalCreatedBefore] = useState<string | null> (null)
   const [originalCreatedFrom, setOriginalCreatedFrom] = useState<string | null> (null)
   const [parentPostIds, setParentPostIds] = useState ('')
@@ -43,6 +53,8 @@ const PostNewPage: FC<Props> = ({ user }) => {
   const thumbnailPreviewRef = useRef ('')
 
   const handleSubmit = async () => {
+    clearValidationErrors ()
+
     const formData = new FormData
     formData.append ('title', title)
     formData.append ('url', url)
@@ -61,8 +73,9 @@ const PostNewPage: FC<Props> = ({ user }) => {
       toast ({ title: '投稿成功!' })
       navigate ('/posts')
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '投稿失敗', description: '入力を確認してください。' })
     }
   }
@@ -126,85 +139,99 @@ const PostNewPage: FC<Props> = ({ user }) => {
 	</Helmet>
 	<Form>
 	  <PageTitle>広場に投稿を追加する</PageTitle>
+	  <FieldError messages={baseErrors}/>
 
 	  {/* URL */}
-	  <div>
-	    <Label>URL</Label>
-	    <input type="url"
-		   placeholder="例：https://www.nicovideo.jp/watch/..."
-		   value={url}
-		   onChange={e => setURL (e.target.value)}
-		   className="w-full border p-2 rounded"
-		   onBlur={handleURLBlur}/>
-	  </div>
+	  <FormField label="URL" messages={fieldErrors.url}>
+	    {({ describedBy, invalid }) => (
+		<input type="url"
+		       placeholder="例：https://www.nicovideo.jp/watch/..."
+		       value={url}
+		       onChange={e => setURL (e.target.value)}
+		       aria-describedby={describedBy}
+		       aria-invalid={invalid}
+		       className={inputClass (invalid)}
+		       onBlur={handleURLBlur}/>)}
+	  </FormField>
 
 	  {/* タイトル */}
-	  <div>
-	    <Label checkBox={{
-		label:    '自動',
-		checked:  titleAutoFlg,
-		onChange: ev => setTitleAutoFlg (ev.target.checked)}}>
-	      タイトル
-	    </Label>
-	    <input type="text"
-		   className="w-full border rounded p-2"
-		   value={title}
-		   placeholder={titleLoading ? 'Loading...' : ''}
-		   onChange={ev => setTitle (ev.target.value)}
-		   disabled={titleAutoFlg}/>
-	  </div>
+	  <FormField
+	      checkBox={{
+		  label:    '自動',
+		  checked:  titleAutoFlg,
+		  onChange: ev => setTitleAutoFlg (ev.target.checked)}}
+	      label="タイトル"
+	      messages={fieldErrors.title}>
+	    {({ describedBy, invalid }) => (
+		<input type="text"
+		       aria-describedby={describedBy}
+		       aria-invalid={invalid}
+		       className={inputClass (invalid)}
+		       value={title}
+		       placeholder={titleLoading ? 'Loading...' : ''}
+		       onChange={ev => setTitle (ev.target.value)}
+		       disabled={titleAutoFlg}/>)}
+	  </FormField>
 
 	  {/* サムネール */}
-	  <div>
-	    <Label checkBox={{
-		label:    '自動',
-		checked:  thumbnailAutoFlg,
-		onChange: ev => setThumbnailAutoFlg (ev.target.checked)}}>
-	      サムネール
-	    </Label>
-	    {thumbnailAutoFlg
-	     ? (thumbnailLoading
-		? <p className="text-gray-500 text-sm">Loading...</p>
-		: !(thumbnailPreview) && (
-		    <p className="text-gray-500 text-sm">
-		      URL から自動取得されます。
-		    </p>))
-	     : (
-		<input type="file"
-		       accept="image/*"
-		       onChange={e => {
-			 const file = e.target.files?.[0]
-			 if (file)
-			   {
-			     setThumbnailFile (file)
-			     setThumbnailPreview (URL.createObjectURL (file))
-			   }
-		       }}/>)}
-	    {thumbnailPreview && (
-		<img src={thumbnailPreview}
-		     alt="preview"
-		     className="mt-2 max-h-48 rounded border"/>)}
-	  </div>
+	  <FormField
+	      checkBox={{
+		  label:    '自動',
+		  checked:  thumbnailAutoFlg,
+		  onChange: ev => setThumbnailAutoFlg (ev.target.checked)}}
+	      label="サムネール"
+	      messages={fieldErrors.thumbnail}>
+	    {({ describedBy, invalid }) => (
+		<>
+		  {thumbnailAutoFlg
+		   ? (thumbnailLoading
+		      ? <p className="text-gray-500 text-sm">Loading...</p>
+		      : !(thumbnailPreview) && (
+			  <p className="text-gray-500 text-sm">
+			    URL から自動取得されます。
+			  </p>))
+		   : (
+		      <input type="file"
+			     accept="image/*"
+			     aria-describedby={describedBy}
+			     aria-invalid={invalid}
+			     onChange={e => {
+			       const file = e.target.files?.[0]
+			       if (file)
+				 {
+				   setThumbnailFile (file)
+				   setThumbnailPreview (URL.createObjectURL (file))
+				 }
+			     }}/>)}
+		  {thumbnailPreview && (
+		      <img src={thumbnailPreview}
+			   alt="preview"
+			   className="mt-2 max-h-48 rounded border"/>)}
+		</>)}
+	  </FormField>
 
 	  {/* 親投稿 */}
-	  <div>
-	    <Label>親投稿</Label>
-	    <input
-		type="text"
-		value={parentPostIds}
-		onChange={e => setParentPostIds (e.target.value)}
-		className="w-full border p-2 rounded"/>
-	  </div>
+	  <FormField label="親投稿" messages={fieldErrors.parentPostIds}>
+	    {({ describedBy, invalid }) => (
+		<input
+		    type="text"
+		    value={parentPostIds}
+		    onChange={e => setParentPostIds (e.target.value)}
+		    aria-describedby={describedBy}
+		    aria-invalid={invalid}
+		    className={inputClass (invalid)}/>)}
+	  </FormField>
 
 	  {/* タグ */}
-	  <PostFormTagsArea tags={tags} setTags={setTags}/>
+	  <PostFormTagsArea tags={tags} setTags={setTags} errors={fieldErrors.tags}/>
 
 	  {/* オリジナルの作成日時 */}
 	  <PostOriginalCreatedTimeField
 	      originalCreatedFrom={originalCreatedFrom}
 	      setOriginalCreatedFrom={setOriginalCreatedFrom}
 	      originalCreatedBefore={originalCreatedBefore}
-	      setOriginalCreatedBefore={setOriginalCreatedBefore}/>
+	      setOriginalCreatedBefore={setOriginalCreatedBefore}
+	      errors={fieldErrors.originalCreatedAt}/>
 
 	  {/* 送信 */}
 	  <Button onClick={handleSubmit}
diff --git a/frontend/src/pages/posts/PostSearchPage.tsx b/frontend/src/pages/posts/PostSearchPage.tsx
index d68909d..7237c9a 100644
--- a/frontend/src/pages/posts/PostSearchPage.tsx
+++ b/frontend/src/pages/posts/PostSearchPage.tsx
@@ -8,7 +8,7 @@ import PrefetchLink from '@/components/PrefetchLink'
 import SortHeader from '@/components/SortHeader'
 import TagLink from '@/components/TagLink'
 import DateTimeField from '@/components/common/DateTimeField'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import Pagination from '@/components/common/Pagination'
 import TagInput from '@/components/common/TagInput'
@@ -16,7 +16,7 @@ import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
 import { fetchPosts } from '@/lib/posts'
 import { postsKeys } from '@/lib/queryKeys'
-import { dateString, originalCreatedAtString } from '@/lib/utils'
+import { dateString, inputClass, originalCreatedAtString } from '@/lib/utils'
 
 import type { FC, FormEvent } from 'react'
 
@@ -138,31 +138,33 @@ const PostSearchPage: FC = () => {
 
 	  <form onSubmit={handleSearch} className="space-y-2">
 	    {/* タイトル */}
-	    <div>
-	      <Label>タイトル</Label>
-	      <input
-		  type="text"
-		  value={title}
-		  onChange={e => setTitle (e.target.value)}
-		  className="w-full border p-2 rounded"/>
-	    </div>
+	    <FormField label="タイトル">
+	      {({ invalid }) => (
+		  <input
+		      type="text"
+		      value={title}
+		      onChange={e => setTitle (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
 
 	    {/* URL */}
-	    <div>
-	      <Label>URL</Label>
-	      <input
-		  type="text"
-		  value={url}
-		  onChange={e => setURL (e.target.value)}
-		  className="w-full border p-2 rounded"/>
-	    </div>
+	    <FormField label="URL">
+	      {({ invalid }) => (
+		  <input
+		      type="text"
+		      value={url}
+		      onChange={e => setURL (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
 
 	    {/* タグ */}
+	    <FormField label="タグ">
+	      {() => (
+		  <TagInput
+		      value={tagsStr}
+		      setValue={setTagsStr}/>)}
+	    </FormField>
 	    <div>
-	      <Label>タグ</Label>
-	      <TagInput
-		  value={tagsStr}
-		  setValue={setTagsStr}/>
 	      <fieldset className="w-full my-2">
 		<label>検索区分：</label>
 		<label className="mx-2">
@@ -185,40 +187,46 @@ const PostSearchPage: FC = () => {
 	    </div>
 
 	    {/* オリジナルの投稿日時 */}
-	    <div>
-	      <Label>オリジナルの投稿日時</Label>
-	      <DateTimeField
-		  value={originalCreatedFrom ?? undefined}
-		  onChange={setOriginalCreatedFrom}/>
-	      <span className="mx-1">〜</span>
-	      <DateTimeField
-		  value={originalCreatedTo ?? undefined}
-		  onChange={setOriginalCreatedTo}/>
-	    </div>
+	    <FormField label="オリジナルの投稿日時">
+	      {() => (
+		  <>
+		    <DateTimeField
+			value={originalCreatedFrom ?? undefined}
+			onChange={setOriginalCreatedFrom}/>
+		    <span className="mx-1">〜</span>
+		    <DateTimeField
+			value={originalCreatedTo ?? undefined}
+			onChange={setOriginalCreatedTo}/>
+		  </>)}
+	    </FormField>
 
 	    {/* 投稿日時 */}
-	    <div>
-	      <Label>投稿日時</Label>
-	      <DateTimeField
-		  value={createdFrom ?? undefined}
-		  onChange={setCreatedFrom}/>
-	      <span className="mx-1">〜</span>
-	      <DateTimeField
-		  value={createdTo ?? undefined}
-		  onChange={setCreatedTo}/>
-	    </div>
+	    <FormField label="投稿日時">
+	      {() => (
+		  <>
+		    <DateTimeField
+			value={createdFrom ?? undefined}
+			onChange={setCreatedFrom}/>
+		    <span className="mx-1">〜</span>
+		    <DateTimeField
+			value={createdTo ?? undefined}
+			onChange={setCreatedTo}/>
+		  </>)}
+	    </FormField>
 
 	    {/* 更新日時 */}
-	    <div>
-	      <Label>更新日時</Label>
-	      <DateTimeField
-		  value={updatedFrom ?? undefined}
-		  onChange={setUpdatedFrom}/>
-	      <span className="mx-1">〜</span>
-	      <DateTimeField
-		  value={updatedTo ?? undefined}
-		  onChange={setUpdatedTo}/>
-	    </div>
+	    <FormField label="更新日時">
+	      {() => (
+		  <>
+		    <DateTimeField
+			value={updatedFrom ?? undefined}
+			onChange={setUpdatedFrom}/>
+		    <span className="mx-1">〜</span>
+		    <DateTimeField
+			value={updatedTo ?? undefined}
+			onChange={setUpdatedTo}/>
+		  </>)}
+	    </FormField>
 
 	    {/* 検索 */}
 	    <div className="py-3">
diff --git a/frontend/src/pages/tags/NicoTagListPage.test.tsx b/frontend/src/pages/tags/NicoTagListPage.test.tsx
new file mode 100644
index 0000000..c5efcd3
--- /dev/null
+++ b/frontend/src/pages/tags/NicoTagListPage.test.tsx
@@ -0,0 +1,273 @@
+import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import NicoTagListPage from '@/pages/tags/NicoTagListPage'
+import { dateString } from '@/lib/utils'
+import { buildTag, buildUser } from '@/test/factories'
+import { renderWithProviders } from '@/test/render'
+
+import type { NicoTag } from '@/types'
+
+const api = vi.hoisted (() => ({
+  apiGet:     vi.fn (),
+  apiPut:     vi.fn (),
+  isApiError: vi.fn (),
+}))
+
+const toastApi = vi.hoisted (() => ({
+  toast: vi.fn (),
+}))
+
+const dialogue = vi.hoisted (() => ({
+  confirm: vi.fn (),
+}))
+
+const scrollIntoView = vi.fn ()
+
+vi.mock ('@/lib/api', () => api)
+vi.mock ('@/components/ui/use-toast', () => toastApi)
+vi.mock ('@/components/dialogues/DialogueProvider', () => ({
+  useDialogue: () => dialogue,
+}))
+
+const buildNicoTag = (values: Partial<NicoTag> = {}): NicoTag => ({
+    ...buildTag (),
+    ...values,
+    category:               'nico',
+    linkedTags:             values.linkedTags ?? [],
+    recentPostTagCreatedAt: values.recentPostTagCreatedAt ?? null,
+})
+
+const renderPage = (route = '/tags/nico') =>
+  renderWithProviders (
+      <NicoTagListPage user={buildUser ({ role: 'member' })}/>,
+      { route },
+  )
+
+describe ('NicoTagListPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.isApiError.mockReturnValue (false)
+    dialogue.confirm.mockResolvedValue (true)
+    Element.prototype.scrollIntoView = scrollIntoView
+    scrollIntoView.mockClear ()
+  })
+
+  it ('loads a filtered page from URL search parameters', async () => {
+    api.apiGet.mockResolvedValue ({
+      tags:  [buildNicoTag ({
+        id:                     1,
+        name:                   'nico:linked',
+        createdAt:              '2024-01-02T03:04:05Z',
+        recentPostTagCreatedAt: '2025-01-02T03:04:05Z',
+        updatedAt:              '2026-01-02T03:04:05Z',
+      })],
+      count: 21,
+    })
+
+    renderPage (
+        '/tags/nico?name=linked&linked_tag=destination&link_status=linked'
+	+ '&page=2&order=name:asc',
+    )
+
+    await waitFor (() => {
+      expect (api.apiGet).toHaveBeenCalledWith (
+          '/tags/nico',
+          { params: {
+            page:        2,
+            limit:       20,
+            name:        'linked',
+            linked_tag:  'destination',
+            link_status: 'linked',
+            order:       'name:asc',
+          } },
+      )
+    })
+    expect (await screen.findByText ('21 件')).toBeInTheDocument ()
+    expect (screen.getByLabelText ('前のページ')).toBeInTheDocument ()
+    expect (screen.queryByText ('なし')).not.toBeInTheDocument ()
+    expect (screen.getByText (dateString ('2025-01-02T03:04:05Z'))).toBeInTheDocument ()
+    expect (screen.queryByText (dateString ('2026-01-02T03:04:05Z'))).not.toBeInTheDocument ()
+    expect (screen.getByRole ('link', { name: 'ニコニコタグ ▲' })).toHaveAttribute (
+        'href',
+        expect.stringContaining ('order=name%3Adesc'),
+    )
+    expect (screen.getByRole ('link', { name: '最初に記載された日時' })).toHaveAttribute (
+        'href',
+        expect.stringContaining ('order=created_at%3Adesc'),
+    )
+    expect (screen.getByRole ('link', { name: '最近記載された日時' })).toHaveAttribute (
+        'href',
+        expect.stringContaining ('order=updated_at%3Adesc'),
+    )
+
+    fireEvent.mouseEnter (screen.getByLabelText ('前のページ'))
+    await waitFor (() => {
+      expect (api.apiGet).toHaveBeenLastCalledWith (
+          '/tags/nico',
+          { params: {
+            page:        1,
+            limit:       20,
+            name:        'linked',
+            linked_tag:  'destination',
+            link_status: 'linked',
+            order:       'name:asc',
+          } },
+      )
+    })
+  })
+
+  it ('scrolls to the table when moving between pages', async () => {
+    api.apiGet.mockResolvedValue ({
+      tags:  [buildNicoTag ({ id: 1, name: 'nico:linked' })],
+      count: 21,
+    })
+
+    renderPage ()
+
+    fireEvent.click (await screen.findByLabelText ('次のページ'))
+
+    await waitFor (() => {
+      expect (scrollIntoView).toHaveBeenCalledWith ({ behavior: 'smooth' })
+    })
+  })
+
+  it ('navigates with submitted search conditions', async () => {
+    api.apiGet.mockResolvedValue ({ tags: [], count: 0 })
+    renderPage ()
+
+    fireEvent.change (screen.getByLabelText ('ニコニコタグ'), {
+      target: { value: 'source' },
+    })
+    fireEvent.change (screen.getByLabelText ('連携タグ'), {
+      target: { value: 'destination' },
+    })
+    fireEvent.change (screen.getByLabelText ('連携状態'), {
+      target: { value: 'unlinked' },
+    })
+    fireEvent.submit (screen.getByRole ('button', { name: '検索' }).closest ('form')!)
+
+    await waitFor (() => {
+      expect (api.apiGet).toHaveBeenLastCalledWith (
+          '/tags/nico',
+          { params: expect.objectContaining ({
+            page:        1,
+            name:        'source',
+            linked_tag:  'destination',
+            link_status: 'unlinked',
+          }) },
+      )
+    })
+  })
+
+  it ('updates links from a tag card', async () => {
+    api.apiGet
+      .mockResolvedValueOnce ({
+        tags:  [buildNicoTag ({ id: 7, name: 'nico:source' })],
+        count: 1,
+      })
+      .mockResolvedValueOnce ({
+        tags: [
+          buildNicoTag ({
+            id:         7,
+            name:       'nico:source',
+            linkedTags: [buildTag ({ id: 8, name: '連携先' })],
+          }),
+        ],
+        count: 1,
+      })
+    api.apiPut.mockResolvedValueOnce ([buildTag ({ id: 8, name: '連携先' })])
+
+    renderPage ()
+
+    fireEvent.click (await screen.findByRole ('button', { name: '編集' }))
+    fireEvent.change (screen.getByLabelText ('連携する広場タグ'), {
+      target: { value: '連携先' },
+    })
+    fireEvent.click (screen.getByRole ('button', { name: '保存' }))
+
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledWith (
+          '/tags/nico/7',
+          expect.any (FormData),
+          { headers: { 'Content-Type': 'multipart/form-data' } },
+      )
+    })
+    expect (await screen.findByText ('1 件')).toBeInTheDocument ()
+  })
+
+  it ('asks before discarding changes when editing another tag', async () => {
+    api.apiGet.mockResolvedValueOnce ({
+      tags: [
+        buildNicoTag ({ id: 1, name: 'nico:first' }),
+        buildNicoTag ({ id: 2, name: 'nico:second' }),
+      ],
+      count: 2,
+    })
+
+    renderPage ()
+    dialogue.confirm.mockResolvedValueOnce (false)
+
+    const editButtons = await screen.findAllByRole ('button', { name: '編集' })
+    fireEvent.click (editButtons[0])
+    fireEvent.change (screen.getByLabelText ('連携する広場タグ'), {
+      target: { value: '入力中' },
+    })
+    fireEvent.click (screen.getAllByRole ('button', { name: '編集' })[0])
+
+    await waitFor (() => {
+      expect (dialogue.confirm).toHaveBeenCalledWith ({
+        title:       '編集中の内容を破棄しますか?',
+        confirmText: '破棄',
+        variant:     'danger',
+      })
+    })
+    expect (screen.getAllByLabelText ('連携する広場タグ')).toHaveLength (1)
+    expect (screen.getByLabelText ('連携する広場タグ')).toHaveValue ('入力中')
+  })
+
+  it ('switches editing rows without confirmation when unchanged', async () => {
+    api.apiGet.mockResolvedValueOnce ({
+      tags: [
+        buildNicoTag ({ id: 1, name: 'nico:first' }),
+        buildNicoTag ({ id: 2, name: 'nico:second' }),
+      ],
+      count: 2,
+    })
+
+    renderPage ()
+
+    const editButtons = await screen.findAllByRole ('button', { name: '編集' })
+    fireEvent.click (editButtons[0])
+    fireEvent.click (screen.getAllByRole ('button', { name: '編集' })[0])
+
+    expect (dialogue.confirm).not.toHaveBeenCalled ()
+    expect (screen.getAllByLabelText ('連携する広場タグ')).toHaveLength (1)
+  })
+
+  it ('shows tags field validation errors inside the edited card', async () => {
+    api.apiGet.mockResolvedValueOnce ({
+      tags:  [buildNicoTag ({ id: 7, name: 'nico:source' })],
+      count: 1,
+    })
+    api.isApiError.mockReturnValue (true)
+    api.apiPut.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          errors:      { tags: ['タグ名を確認してください．'] },
+          base_errors: [],
+        },
+      },
+    })
+
+    renderPage ()
+
+    fireEvent.click (await screen.findByRole ('button', { name: '編集' }))
+    fireEvent.click (screen.getByRole ('button', { name: '保存' }))
+
+    expect (await screen.findByText ('タグ名を確認してください．')).toBeInTheDocument ()
+    expect (screen.getByLabelText ('連携する広場タグ')).toHaveAttribute ('aria-invalid', 'true')
+  })
+})
diff --git a/frontend/src/pages/tags/NicoTagListPage.tsx b/frontend/src/pages/tags/NicoTagListPage.tsx
index e3c45bc..39ebd93 100644
--- a/frontend/src/pages/tags/NicoTagListPage.tsx
+++ b/frontend/src/pages/tags/NicoTagListPage.tsx
@@ -1,104 +1,179 @@
-import type { FC } from 'react'
-
-import { useCallback, useEffect, useRef, useState } from 'react'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
+import { Check, LoaderCircle, Pencil, X } from 'lucide-react'
+import { useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
+import { useLocation, useNavigate } from 'react-router-dom'
 
 import TagLink from '@/components/TagLink'
+import SortHeader from '@/components/SortHeader'
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
+import Pagination from '@/components/common/Pagination'
 import TextArea from '@/components/common/TextArea'
+import { useDialogue } from '@/components/dialogues/DialogueProvider'
 import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
-import { apiGet, apiPut } from '@/lib/api'
+import { apiPut } from '@/lib/api'
+import { extractValidationError } from '@/lib/apiErrors'
+import { tagsKeys } from '@/lib/queryKeys'
+import { fetchNicoTags } from '@/lib/tags'
+import { cn, dateString, inputClass } from '@/lib/utils'
+import { canEditContent } from '@/lib/users'
 
-import type { NicoTag, Tag, User } from '@/types'
+import type { FC, FormEvent } from 'react'
 
+import type { FetchNicoTagsOrder, FetchNicoTagsOrderField, NicoTag, Tag, User } from '@/types'
+
+type LinkStatus = 'all' | 'linked' | 'unlinked'
 type Props = { user: User | null }
 
 
+const setIf = (qs: URLSearchParams, key: string, value: string) => {
+  const trimmed = value.trim ()
+  if (trimmed)
+    qs.set (key, trimmed)
+}
+
+
 const NicoTagListPage: FC<Props> = ({ user }) => {
-  const [cursor, setCursor] = useState ('')
-  const [editing, setEditing] = useState<{ [key: number]: boolean }> ({ })
-  const [loading, setLoading] = useState (false)
-  const [nicoTags, setNicoTags] = useState<NicoTag[]> ([])
-  const [rawTags, setRawTags] = useState<{ [key: number]: string }> ({ })
+  const dialogue = useDialogue ()
+  const location = useLocation ()
+  const navigate = useNavigate ()
+  const queryClient = useQueryClient ()
+  const query = useMemo (() => new URLSearchParams (location.search), [location.search])
 
-  const loaderRef = useRef<HTMLDivElement | null> (null)
+  const page = Number (query.get ('page') ?? 1)
+  const limit = Number (query.get ('limit') ?? 20)
+  const qName = query.get ('name') ?? ''
+  const qLinkedTag = query.get ('linked_tag') ?? ''
+  const qLinkStatus = (query.get ('link_status') || 'all') as LinkStatus
+  const order = (query.get ('order') || 'updated_at:desc') as FetchNicoTagsOrder
 
-  const memberFlg = ['admin', 'member'].some (r => user?.role === r)
+  const [editingId, setEditingId] = useState<number | null> (null)
+  const [errorsByTagId, setErrorsByTagId] = useState<Record<number, string[]>> ({ })
+  const [linkStatus, setLinkStatus] = useState<LinkStatus> ('all')
+  const [linkedTag, setLinkedTag] = useState ('')
+  const [name, setName] = useState ('')
+  const [rawTags, setRawTags] = useState<Record<number, string>> ({ })
+  const [savingId, setSavingId] = useState<number | null> (null)
 
-  const applyLoadedTags = useCallback ((data: { tags: NicoTag[]; nextCursor: string },
-					withCursor: boolean) => {
-    setNicoTags (tags => [...(withCursor ? tags : []), ...data.tags])
-    setCursor (data.nextCursor)
+  const keys = {
+      name: qName, linkedTag: qLinkedTag, linkStatus: qLinkStatus, page, limit, order }
+  const { data, isError, isLoading: loading } = useQuery ({
+      queryKey: tagsKeys.nicoIndex (keys),
+      queryFn:  () => fetchNicoTags (keys) })
+  const nicoTags = data?.tags ?? []
+  const count = data?.count ?? 0
 
-    const newEditing = Object.fromEntries (data.tags.map (t => [t.id, false]))
-    setEditing (editing => ({ ...editing, ...newEditing }))
+  const editable = canEditContent (user)
+  const totalPages = Math.ceil (count / limit)
 
-    const newRawTags = Object.fromEntries (
-	data.tags.map (t => [t.id, t.linkedTags.map (lt => lt.name).join (' ')]))
-    setRawTags (rawTags => ({ ...rawTags, ...newRawTags }))
-  }, [])
+  const handleSearch = (e: FormEvent) => {
+    e.preventDefault ()
 
-  const loadInitial = useCallback (async () => {
-    setLoading (true)
-
-    const data = await apiGet<{ tags: NicoTag[]; nextCursor: string }> ('/tags/nico')
-    applyLoadedTags (data, false)
-
-    setLoading (false)
-  }, [applyLoadedTags])
-
-  const loadMore = useCallback (async () => {
-    setLoading (true)
-
-    const data = await apiGet<{ tags: NicoTag[]; nextCursor: string }> (
-	'/tags/nico', { params: { cursor } })
-    applyLoadedTags (data, true)
-
-    setLoading (false)
-  }, [applyLoadedTags, cursor])
-
-  const handleEdit = async (id: number) => {
-    if (editing[id])
-      {
-	const formData = new FormData
-	formData.append ('tags', rawTags[id])
-
-	const data = await apiPut<Tag[]> (`/tags/nico/${ id }`, formData,
-				    { headers: { 'Content-Type': 'multipart/form-data' } })
-	setNicoTags (nicoTags => {
-	  nicoTags.find (t => t.id === id)!.linkedTags = data
-	  return [...nicoTags]
-	})
-	setRawTags (rawTags => ({ ...rawTags, [id]: data.map (t => t.name).join (' ') }))
-
-	toast ({ title: '更新しました．' })
-      }
-
-    setEditing (editing => ({ ...editing, [id]: !(editing[id]) }))
+    const qs = new URLSearchParams ()
+    setIf (qs, 'name', name)
+    setIf (qs, 'linked_tag', linkedTag)
+    if (linkStatus !== 'all')
+      qs.set ('link_status', linkStatus)
+    qs.set ('page', '1')
+    qs.set ('limit', String (limit))
+    qs.set ('order', order)
+    navigate (`${ location.pathname }?${ qs.toString () }`)
   }
 
-  useEffect(() => {
-    const observer = new IntersectionObserver (entries => {
-      if (entries[0].isIntersecting && !(loading) && cursor)
-	loadMore ()
-    }, { threshold: 1 })
+  const defaultDirection = {
+      name:       'asc',
+      created_at: 'desc',
+      updated_at: 'desc',
+  } as const
 
-    const target = loaderRef.current
-    if (target)
-      observer.observe (target)
+  const beginEdit = async (tag: NicoTag) => {
+    const editingTag = nicoTags.find (tag => tag.id === editingId)
+    const editingValue = editingTag?.linkedTags.map (tag => tag.name).join (' ') ?? ''
+    const editingChanged = editingId != null && rawTags[editingId] !== editingValue
 
-    return () => {
-      if (target)
-	observer.unobserve (target)
+    if (editingId != null && editingId !== tag.id && editingChanged
+	&& !(await dialogue.confirm ({
+	  title:       '編集中の内容を破棄しますか?',
+	  confirmText: '破棄',
+	  variant:     'danger',
+	})))
+      return
+
+    setEditingId (tag.id)
+    setRawTags (rawTags => ({
+	...rawTags,
+	[tag.id]: tag.linkedTags.map (linkedTag => linkedTag.name).join (' '),
+    }))
+    setErrorsByTagId (errors => ({ ...errors, [tag.id]: [] }))
+  }
+
+  const cancelEdit = (tag: NicoTag) => {
+    setEditingId (null)
+    setRawTags (rawTags => ({
+	...rawTags,
+	[tag.id]: tag.linkedTags.map (linkedTag => linkedTag.name).join (' '),
+    }))
+    setErrorsByTagId (errors => ({ ...errors, [tag.id]: [] }))
+  }
+
+  const saveLinks = async (id: number) => {
+    const formData = new FormData
+    formData.append ('tags', rawTags[id] ?? '')
+    setSavingId (id)
+
+    try
+    {
+      await apiPut<Tag[]> (`/tags/nico/${ id }`, formData,
+			   { headers: { 'Content-Type': 'multipart/form-data' } })
+      setErrorsByTagId (errors => ({ ...errors, [id]: [] }))
+      setEditingId (null)
+      await queryClient.invalidateQueries ({ queryKey: tagsKeys.nicoRoot })
+      toast ({ description: '連携を更新しました．' })
     }
-  }, [cursor, loadMore, loading])
+    catch (e)
+    {
+      const validationError = extractValidationError<'tags'> (e)
+      setErrorsByTagId (errors => ({
+	  ...errors,
+	  [id]: validationError?.fieldErrors.tags
+	      ?? validationError?.baseErrors
+	      ?? ['更新できませんでした．'],
+      }))
+      toast ({ title: '更新失敗', description: '入力内容を確認してください．' })
+    }
+    finally
+    {
+      setSavingId (null)
+    }
+  }
 
   useEffect (() => {
-    setNicoTags ([])
-    loadInitial ()
-  }, [loadInitial])
+    setName (qName)
+    setLinkedTag (qLinkedTag)
+    setLinkStatus (qLinkStatus)
+    setEditingId (null)
+
+    document.querySelector ('table')?.scrollIntoView ({ behavior: 'smooth' })
+  }, [location.search, qLinkedTag, qLinkStatus, qName])
+
+  useEffect (() => {
+    if (!(data))
+      return
+
+    setRawTags (Object.fromEntries (data.tags.map (tag => [
+      tag.id,
+      tag.linkedTags.map (linkedTag => linkedTag.name).join (' '),
+    ])))
+  }, [data])
+
+  useEffect (() => {
+    if (isError)
+      toast ({ title: '読込失敗', description: 'ニコニコ連携を読み込めませんでした．' })
+  }, [isError])
 
   return (
       <MainArea>
@@ -108,58 +183,201 @@ const NicoTagListPage: FC<Props> = ({ user }) => {
 
 	<div className="max-w-xl">
 	  <PageTitle>ニコニコ連携</PageTitle>
+	  <p className="mb-4 text-sm text-gray-600 dark:text-gray-300">
+	    ニコニコタグを広場のタグへ結び付けます．
+	  </p>
+
+	  <form onSubmit={handleSearch} className="space-y-2">
+	    <FormField label="ニコニコタグ">
+	      {({ invalid }) => (
+		  <input
+		      type="text"
+		      aria-label="ニコニコタグ"
+		      value={name}
+		      onChange={e => setName (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
+
+	    <FormField label="連携タグ">
+	      {({ invalid }) => (
+		  <input
+		      type="text"
+		      aria-label="連携タグ"
+		      value={linkedTag}
+		      onChange={e => setLinkedTag (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
+
+	    <FormField label="連携状態">
+	      {({ invalid }) => (
+		  <select
+		      aria-label="連携状態"
+		      value={linkStatus}
+		      onChange={e => setLinkStatus (e.target.value as LinkStatus)}
+		      className={inputClass (invalid)}>
+		    <option value="all">すべて</option>
+		    <option value="linked">連携済み</option>
+		    <option value="unlinked">未連携</option>
+		  </select>)}
+	    </FormField>
+
+	    <div className="py-3">
+	      <button
+		  type="submit"
+		  className="rounded bg-blue-500 px-4 py-2 text-white">
+		検索
+	      </button>
+	    </div>
+	  </form>
 	</div>
 
-	<div className="mt-4">
-	  {nicoTags.length > 0 && (
-	      <table className="table-auto w-full border-collapse mb-4">
-		<thead className="border-b-2 border-black dark:border-white">
-		  <tr>
-		    <th className="p-2 text-left">ニコニコタグ</th>
-		    <th className="p-2 text-left">連携タグ</th>
-		    {memberFlg && <th></th>}
-		  </tr>
-		</thead>
-		<tbody>
-		  {nicoTags.map ((tag, i) => (
-		      <tr key={i} className="even:bg-gray-100 dark:even:bg-gray-700">
-			<td className="p-2">
-			  <TagLink tag={tag} withWiki={false} withCount={false}/>
-			</td>
-			<td className="p-2">
-			  {editing[tag.id]
-			   ? (
-			      <TextArea value={rawTags[tag.id]} onChange={ev => {
-				setRawTags (rawTags => ({ ...rawTags, [tag.id]: ev.target.value }))
-			      }}/>)
-			   : tag.linkedTags.map((lt, j) => (
-			      <span key={j} className="mr-2">
-				<TagLink tag={lt}
-					 linkFlg={false}
-					 withCount={false}/>
-			      </span>))}
-			</td>
-			{memberFlg && (
-			    <td className="p-2">
-			      <a href="#" onClick={ev => {
-				ev.preventDefault ()
-				handleEdit (tag.id)
-			      }}>
-			        {editing[tag.id]
-				 ? (
-				    <span className="text-red-600 hover:text-red-400
-						     dark:text-red-300 dark:hover:text-red-100">
-				      更新
-				    </span>)
-				 : <span>編輯</span>}
-			      </a>
-			    </td>)}
-		      </tr>))}
-		</tbody>
-	      </table>)}
-	  {loading && 'Loading...'}
-	  <div ref={loaderRef} className="h-12"></div>
-	</div>
+	{loading
+	 ? 'Loading...'
+	 : (
+	    <div className="mt-6">
+	      <div className="mb-3 flex items-baseline justify-between gap-4">
+		<h2 className="text-lg font-bold">検索結果</h2>
+		<span className="text-sm text-gray-500 dark:text-gray-400">{count} 件</span>
+	      </div>
+
+	      {nicoTags.length > 0
+	       ? (
+		  <div className="overflow-x-auto">
+		    <table className="w-full min-w-[800px] table-fixed border-collapse">
+		      <colgroup>
+			<col className="w-64"/>
+			<col className="w-[48rem]"/>
+			<col className="w-56"/>
+			<col className="w-56"/>
+			{editable && <col className="w-24"/>}
+		      </colgroup>
+		      <thead className="border-b-2 border-black dark:border-white">
+			<tr>
+			  <th className="p-2 text-left whitespace-nowrap">
+			    <SortHeader<FetchNicoTagsOrderField>
+				by="name"
+				label="ニコニコタグ"
+				currentOrder={order}
+				defaultDirection={defaultDirection}/>
+			  </th>
+			  <th className="p-2 text-left">連携タグ</th>
+			  <th className="p-2 text-left whitespace-nowrap">
+			    <SortHeader<FetchNicoTagsOrderField>
+				by="created_at"
+				label="最初に記載された日時"
+				currentOrder={order}
+				defaultDirection={defaultDirection}/>
+			  </th>
+			  <th className="p-2 text-left whitespace-nowrap">
+			    <SortHeader<FetchNicoTagsOrderField>
+				by="updated_at"
+				label="最近記載された日時"
+				currentOrder={order}
+				defaultDirection={defaultDirection}/>
+			  </th>
+			  {editable && <th className="p-2"></th>}
+			</tr>
+		      </thead>
+		      <tbody>
+			{nicoTags.map (tag => {
+			  const isEditing = editingId === tag.id
+
+			  return [
+			    <tr
+				key={tag.id}
+				className={cn (
+				    'border-b border-gray-200 dark:border-gray-700',
+				    isEditing
+				    ? 'bg-rose-50 dark:bg-rose-950/30'
+				    : 'even:bg-gray-100 dark:even:bg-gray-800')}>
+			      <td className="p-2 align-top font-semibold">
+				<TagLink tag={tag} withWiki={false} withCount={false}/>
+			      </td>
+			      <td className="p-2 align-top">
+				{tag.linkedTags.map ((linkedTag, i) => (
+				    <span key={linkedTag.id}>
+				      {i > 0 && ' '}
+				      <TagLink
+					  tag={linkedTag}
+					  linkFlg={false}
+					  withCount={false}/>
+				    </span>))}
+			      </td>
+			      <td className="p-2 align-top whitespace-nowrap">
+				{dateString (tag.createdAt)}
+			      </td>
+			      <td className="p-2 align-top whitespace-nowrap">
+				{tag.recentPostTagCreatedAt && dateString (tag.recentPostTagCreatedAt)}
+			      </td>
+			      {editable && (
+				  <td className="p-2 text-right align-top">
+				    {!(isEditing) && (
+					<button
+					    type="button"
+					    onClick={() => beginEdit (tag)}
+					    className="inline-flex items-center gap-1 text-sm text-blue-700
+						       hover:underline dark:text-blue-300">
+					  <Pencil className="size-3.5"/>
+					  編集
+					</button>)}
+				  </td>)}
+			    </tr>,
+			    isEditing && (
+				<tr key={`${ tag.id }-edit`}
+				    className="border-b border-rose-200 bg-rose-50 dark:border-rose-900
+					       dark:bg-rose-950/30">
+				  <td colSpan={editable ? 5 : 4} className="p-3">
+				    <div className="space-y-2">
+				      <label htmlFor={`nico-links-${ tag.id }`}
+					     className="block text-sm font-semibold">
+					連携する広場タグ
+				      </label>
+				      <TextArea
+					  id={`nico-links-${ tag.id }`}
+					  value={rawTags[tag.id] ?? ''}
+					  invalid={(errorsByTagId[tag.id] ?? []).length > 0}
+					  className="min-h-24 resize-y"
+					  placeholder="タグ名を空白または改行で区切って入力"
+					  onChange={e => setRawTags (rawTags => ({
+					      ...rawTags,
+					      [tag.id]: e.target.value,
+					  }))}/>
+				      <FieldError messages={errorsByTagId[tag.id]}/>
+				      <div className="flex justify-end gap-2">
+					<button
+					    type="button"
+					    disabled={savingId === tag.id}
+					    onClick={() => cancelEdit (tag)}
+					    className="inline-flex items-center gap-1 rounded border
+						       border-gray-300 px-3 py-1.5 text-sm
+						       disabled:opacity-50 dark:border-gray-700">
+					  <X className="size-3.5"/>
+					  取消
+					</button>
+					<button
+					    type="button"
+					    disabled={savingId === tag.id}
+					    onClick={() => saveLinks (tag.id)}
+					    className="inline-flex items-center gap-1 rounded bg-rose-700
+						       px-3 py-1.5 text-sm text-white disabled:opacity-50">
+					  {savingId === tag.id
+					   ? <LoaderCircle className="size-3.5 animate-spin"/>
+					   : <Check className="size-3.5"/>}
+					  保存
+					</button>
+				      </div>
+				    </div>
+				  </td>
+				</tr>),
+			  ]
+			})}
+		      </tbody>
+		    </table>
+		  </div>)
+	       : <p>結果ないよ（笑）</p>}
+
+	      <Pagination page={page} totalPages={totalPages}/>
+	    </div>)}
       </MainArea>)
 }
 
diff --git a/frontend/src/pages/tags/TagDetailPage.test.tsx b/frontend/src/pages/tags/TagDetailPage.test.tsx
index e5380cc..9b9ca2c 100644
--- a/frontend/src/pages/tags/TagDetailPage.test.tsx
+++ b/frontend/src/pages/tags/TagDetailPage.test.tsx
@@ -1,6 +1,6 @@
 import { fireEvent, screen, waitFor } from '@testing-library/react'
 import { Route, Routes } from 'react-router-dom'
-import { describe, expect, it, vi } from 'vitest'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import TagDetailPage from '@/pages/tags/TagDetailPage'
 import { buildTag } from '@/test/factories'
@@ -11,7 +11,8 @@ const tagsApi = vi.hoisted (() => ({
 }))
 
 const api = vi.hoisted (() => ({
-  apiPut: vi.fn (),
+  apiPut:     vi.fn (),
+  isApiError: vi.fn (),
 }))
 
 const toastApi = vi.hoisted (() => ({
@@ -28,9 +29,14 @@ const renderPage = () =>
         <Route path="/tags/:id" element={<TagDetailPage/>}/>
       </Routes>,
       { route: '/tags/7' },
-  )
+)
 
 describe ('TagDetailPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.isApiError.mockReturnValue (false)
+  })
+
   it ('loads and displays an editable tag', async () => {
     tagsApi.fetchTag.mockResolvedValueOnce (
         buildTag ({ id: 7, name: '虹夏', category: 'character', aliases: ['drums'] }),
@@ -68,4 +74,28 @@ describe ('TagDetailPage', () => {
 
     expect (await screen.findByRole ('button', { name: '更新' })).toBeDisabled ()
   })
+
+  it ('shows validation errors returned for tag fields', async () => {
+    tagsApi.fetchTag.mockResolvedValueOnce (buildTag ({ id: 7, name: 'old' }))
+    api.isApiError.mockReturnValue (true)
+    api.apiPut.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      { category: ['ニコタグは変更できません．'] },
+          base_errors: [],
+        },
+      },
+    })
+
+    renderPage ()
+
+    await screen.findByDisplayValue ('old')
+    fireEvent.submit (screen.getByRole ('button', { name: '更新' }).closest ('form')!)
+
+    expect (await screen.findByText ('ニコタグは変更できません．')).toBeInTheDocument ()
+    expect (screen.getByRole ('combobox')).toHaveAttribute ('aria-invalid', 'true')
+  })
 })
diff --git a/frontend/src/pages/tags/TagDetailPage.tsx b/frontend/src/pages/tags/TagDetailPage.tsx
index fc10727..386208e 100644
--- a/frontend/src/pages/tags/TagDetailPage.tsx
+++ b/frontend/src/pages/tags/TagDetailPage.tsx
@@ -3,7 +3,8 @@ import { useEffect, useState } from 'react'
 import { useParams } from 'react-router-dom'
 
 import TagLink from '@/components/TagLink'
-import Label from '@/components/common/Label'
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
@@ -11,12 +12,15 @@ import { CATEGORIES, CATEGORY_NAMES } from '@/consts'
 import { apiPut } from '@/lib/api'
 import { postsKeys, tagsKeys } from '@/lib/queryKeys'
 import { fetchTag } from '@/lib/tags'
-import { cn } from '@/lib/utils'
+import { cn, inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC, FormEvent } from 'react'
 
 import type { Category, Tag } from '@/types'
 
+type TagFormField = 'name' | 'category' | 'aliases' | 'parentTags'
+
 
 const TagDetailPage: FC = () => {
   const { id } = useParams ()
@@ -32,11 +36,14 @@ const TagDetailPage: FC = () => {
   const [aliases, setAliases] = useState ('')
   const [parentTags, setParentTags] = useState ('')
   const [disabled, setDisabled] = useState (true)
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<TagFormField> ()
 
   const qc = useQueryClient ()
 
   const handleSubmit = async (e: FormEvent) => {
     e.preventDefault ()
+    clearValidationErrors ()
 
     const formData = new FormData
     formData.append ('name', name)
@@ -57,8 +64,9 @@ const TagDetailPage: FC = () => {
       qc.invalidateQueries ({ queryKey: tagsKeys.root })
       toast ({ description: '更新しました．' })
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ description: '更新に失敗しました．' })
     }
   }
@@ -89,57 +97,73 @@ const TagDetailPage: FC = () => {
 	      </PageTitle>
 
 	      <form onSubmit={handleSubmit} className="my-4 space-y-2">
+		<FieldError messages={baseErrors}/>
+
 		{/* 名称 */}
-		<div>
-		  <Label>名称</Label>
-		  {/* TODO: 補完に対応させる */}
-		  <input
-		      type="text"
-		      disabled={disabled}
-		      value={name}
-		      onChange={e => setName (e.target.value)}
-		      className="w-full border p-2 rounded"/>
-		</div>
+		<FormField label="名称" messages={fieldErrors.name}>
+		  {({ describedBy, invalid }) => (
+		      <>
+			{/* TODO: 補完に対応させる */}
+			<input
+			    type="text"
+			    disabled={disabled}
+			    value={name}
+			    onChange={e => setName (e.target.value)}
+			    aria-describedby={describedBy}
+			    aria-invalid={invalid}
+			    className={inputClass (invalid)}/>
+		      </>)}
+		</FormField>
 
 		{/* カテゴリ */}
-		<div>
-		  <Label>カテゴリ</Label>
+		<FormField label="カテゴリ" messages={fieldErrors.category}>
+		  {({ describedBy, invalid }) => (
 		  <select
 		      disabled={disabled}
 		      value={category ?? ''}
 		      onChange={e => setCategory(e.target.value as Category)}
-		      className="w-full border p-2 rounded">
+		      aria-describedby={describedBy}
+		      aria-invalid={invalid}
+		      className={inputClass (invalid)}>
 		    {CATEGORIES.filter (cat => tag.category === 'nico' || cat !== 'nico')
 			       .map (cat => (
 			<option key={cat} value={cat}>
 			  {CATEGORY_NAMES[cat]}
 			</option>))}
-		  </select>
-		</div>
+		  </select>)}
+		</FormField>
 
 		{/* 別名 */}
-		<div>
-		  <Label>別名</Label>
-		  {/* TODO: 補完に対応させる */}
-		  <input
-		      type="text"
-		      disabled={disabled}
-		      value={aliases}
-		      onChange={e => setAliases (e.target.value)}
-		      className="w-full border p-2 rounded"/>
-		</div>
+		<FormField label="別名" messages={fieldErrors.aliases}>
+		  {({ describedBy, invalid }) => (
+		      <>
+			{/* TODO: 補完に対応させる */}
+			<input
+			    type="text"
+			    disabled={disabled}
+			    value={aliases}
+			    onChange={e => setAliases (e.target.value)}
+			    aria-describedby={describedBy}
+			    aria-invalid={invalid}
+			    className={inputClass (invalid)}/>
+		      </>)}
+		</FormField>
 
 		{/* 上位タグ */}
-		<div>
-		  <Label>上位タグ</Label>
-		  {/* TODO: 補完に対応させる */}
-		  <input
-		      type="text"
-		      disabled={disabled}
-		      value={parentTags}
-		      onChange={e => setParentTags (e.target.value)}
-		      className="w-full border p-2 rounded"/>
-		</div>
+		<FormField label="上位タグ" messages={fieldErrors.parentTags}>
+		  {({ describedBy, invalid }) => (
+		      <>
+			{/* TODO: 補完に対応させる */}
+			<input
+			    type="text"
+			    disabled={disabled}
+			    value={parentTags}
+			    onChange={e => setParentTags (e.target.value)}
+			    aria-describedby={describedBy}
+			    aria-invalid={invalid}
+			    className={inputClass (invalid)}/>
+		      </>)}
+		</FormField>
 
 		<div className="py-3">
 		  <button
@@ -157,4 +181,4 @@ const TagDetailPage: FC = () => {
       </MainArea>)
 }
 
-export default TagDetailPage
\ No newline at end of file
+export default TagDetailPage
diff --git a/frontend/src/pages/tags/TagListPage.tsx b/frontend/src/pages/tags/TagListPage.tsx
index 7e2b039..a48f377 100644
--- a/frontend/src/pages/tags/TagListPage.tsx
+++ b/frontend/src/pages/tags/TagListPage.tsx
@@ -7,7 +7,7 @@ import PrefetchLink from '@/components/PrefetchLink'
 import SortHeader from '@/components/SortHeader'
 import TagLink from '@/components/TagLink'
 import DateTimeField from '@/components/common/DateTimeField'
-import Label from '@/components/common/Label'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import Pagination from '@/components/common/Pagination'
 import MainArea from '@/components/layout/MainArea'
@@ -15,7 +15,7 @@ import { SITE_TITLE } from '@/config'
 import { CATEGORIES, CATEGORY_NAMES } from '@/consts'
 import { tagsKeys } from '@/lib/queryKeys'
 import { fetchTags } from '@/lib/tags'
-import { dateString } from '@/lib/utils'
+import { dateString, inputClass } from '@/lib/utils'
 
 import type { FC, FormEvent } from 'react'
 
@@ -127,71 +127,79 @@ const TagListPage: FC = () => {
 
 	  <form onSubmit={handleSearch} className="space-y-2">
 	    {/* 名前 */}
-	    <div>
-	      <Label>名前</Label>
-	      <input
-		  type="text"
-		  value={name}
-		  onChange={e => setName (e.target.value)}
-		  className="w-full border p-2 rounded"/>
-	    </div>
+	    <FormField label="名前">
+	      {({ invalid }) => (
+		  <input
+		      type="text"
+		      value={name}
+		      onChange={e => setName (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
 
 	    {/* カテゴリ */}
-	    <div>
-	      <Label>カテゴリ</Label>
-	      <select
-		  value={category ?? ''}
-		  onChange={e => setCategory((e.target.value || null) as Category | null)}
-		  className="w-full border p-2 rounded">
-		<option value="">&nbsp;</option>
-		{CATEGORIES.map (cat => (
-		    <option key={cat} value={cat}>
-		      {CATEGORY_NAMES[cat]}
-		    </option>))}
-	      </select>
-	    </div>
+	    <FormField label="カテゴリ">
+	      {({ invalid }) => (
+		  <select
+		      value={category ?? ''}
+		      onChange={e => setCategory((e.target.value || null) as Category | null)}
+		      className={inputClass (invalid)}>
+		    <option value="">&nbsp;</option>
+		    {CATEGORIES.map (cat => (
+			<option key={cat} value={cat}>
+			  {CATEGORY_NAMES[cat]}
+			</option>))}
+		  </select>)}
+	    </FormField>
 
 	    {/* 広場の投稿数 */}
-	    <div>
-	      <Label>広場の投稿数</Label>
-	      <input
-		  type="number"
-		  min="0"
-		  value={postCountGTE < 0 ? 0 : String (postCountGTE)}
-		  onChange={e => setPostCountGTE (Number (e.target.value || 0))}
-		  className="border rounded p-2"/>
-	      <span className="mx-1">〜</span>
-	      <input
-		  type="number"
-		  min="0"
-		  value={postCountLTE == null ? '' : String (postCountLTE)}
-		  onChange={e => setPostCountLTE (e.target.value ? Number (e.target.value) : null)}
-		  className="border rounded p-2"/>
-	    </div>
+	    <FormField label="広場の投稿数">
+	      {({ invalid }) => (
+		  <>
+		    <input
+			type="number"
+			min="0"
+			value={postCountGTE < 0 ? 0 : String (postCountGTE)}
+			onChange={e => setPostCountGTE (Number (e.target.value || 0))}
+			className={inputClass (invalid, 'w-auto')}/>
+		    <span className="mx-1">〜</span>
+		    <input
+			type="number"
+			min="0"
+			value={postCountLTE == null ? '' : String (postCountLTE)}
+			onChange={e => setPostCountLTE (e.target.value
+							? Number (e.target.value)
+							: null)}
+			className={inputClass (invalid, 'w-auto')}/>
+		  </>)}
+	    </FormField>
 
 	    {/* はじめて記載された日時 */}
-	    <div>
-	      <Label>はじめて記載された日時</Label>
-	      <DateTimeField
-		  value={createdFrom ?? undefined}
-		  onChange={setCreatedFrom}/>
-	      <span className="mx-1">〜</span>
-	      <DateTimeField
-		  value={createdTo ?? undefined}
-		  onChange={setCreatedTo}/>
-	    </div>
+	    <FormField label="はじめて記載された日時">
+	      {() => (
+		  <>
+		    <DateTimeField
+			value={createdFrom ?? undefined}
+			onChange={setCreatedFrom}/>
+		    <span className="mx-1">〜</span>
+		    <DateTimeField
+			value={createdTo ?? undefined}
+			onChange={setCreatedTo}/>
+		  </>)}
+	    </FormField>
 
 	    {/* 定義の更新日時 */}
-	    <div>
-	      <Label>定義の更新日時</Label>
-	      <DateTimeField
-		  value={updatedFrom ?? undefined}
-		  onChange={setUpdatedFrom}/>
-	      <span className="mx-1">〜</span>
-	      <DateTimeField
-		  value={updatedTo ?? undefined}
-		  onChange={setUpdatedTo}/>
-	    </div>
+	    <FormField label="定義の更新日時">
+	      {() => (
+		  <>
+		    <DateTimeField
+			value={updatedFrom ?? undefined}
+			onChange={setUpdatedFrom}/>
+		    <span className="mx-1">〜</span>
+		    <DateTimeField
+			value={updatedTo ?? undefined}
+			onChange={setUpdatedTo}/>
+		  </>)}
+	    </FormField>
 
 	    <div className="py-3">
 	      <button
diff --git a/frontend/src/pages/theatres/TheatreDetailPage.tsx b/frontend/src/pages/theatres/TheatreDetailPage.tsx
index 0b626bd..558b3cd 100644
--- a/frontend/src/pages/theatres/TheatreDetailPage.tsx
+++ b/frontend/src/pages/theatres/TheatreDetailPage.tsx
@@ -6,12 +6,14 @@ import ErrorScreen from '@/components/ErrorScreen'
 import PostEmbed from '@/components/PostEmbed'
 import PrefetchLink from '@/components/PrefetchLink'
 import TagDetailSidebar from '@/components/TagDetailSidebar'
+import FieldError from '@/components/common/FieldError'
 import MainArea from '@/components/layout/MainArea'
 import SidebarComponent from '@/components/layout/SidebarComponent'
 import { SITE_TITLE } from '@/config'
 import { apiGet, apiPatch, apiPost, apiPut, isApiError } from '@/lib/api'
 import { fetchPost } from '@/lib/posts'
-import { dateString } from '@/lib/utils'
+import { dateString, inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { FC } from 'react'
 
@@ -27,6 +29,8 @@ type TheatreInfo = {
     postStartedAt:  string | null
     watchingUsers:  { id: number; name: string }[] }
 
+type TheatreCommentField = 'content'
+
 const INITIAL_THEATRE_INFO =
     { hostFlg: false,
       postId: null,
@@ -53,6 +57,8 @@ const TheatreDetailPage: FC = () => {
   const [theatreInfo, setTheatreInfo] = useState<TheatreInfo> (INITIAL_THEATRE_INFO)
   const [post, setPost] = useState<Post | null> (null)
   const [videoLength, setVideoLength] = useState (0)
+  const { fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<TheatreCommentField> ()
 
   useEffect (() => {
     loadingRef.current = loading
@@ -284,22 +290,28 @@ const TheatreDetailPage: FC = () => {
 		try
 		{
 		  setSending (true)
+		  clearValidationErrors ()
 		  await apiPost (`/theatres/${ id }/comments`, { content })
 		  setContent ('')
 		  commentsRef.current?.scrollTo ({ top: 0, behavior: 'smooth' })
 		}
+		catch (error)
+		{
+		  applyValidationError (error)
+		}
 		finally
 		{
 		  setSending (false)
 		}
 	      }}>
 	    <input
-		className="w-full p-2 border rounded"
+		className={inputClass ((fieldErrors.content ?? []).length > 0)}
 		type="text"
 		placeholder="ここにコメントを入力"
 		value={content}
 		onChange={e => setContent (e.target.value)}
 		disabled={sending}/>
+	    <FieldError messages={fieldErrors.content}/>
 
 	    <div
 		ref={commentsRef}
diff --git a/frontend/src/pages/users/SettingPage.test.tsx b/frontend/src/pages/users/SettingPage.test.tsx
index 3e1a232..0595642 100644
--- a/frontend/src/pages/users/SettingPage.test.tsx
+++ b/frontend/src/pages/users/SettingPage.test.tsx
@@ -1,12 +1,13 @@
 import { fireEvent, screen, waitFor } from '@testing-library/react'
-import { describe, expect, it, vi } from 'vitest'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import SettingPage from '@/pages/users/SettingPage'
 import { buildUser } from '@/test/factories'
 import { renderWithProviders } from '@/test/render'
 
 const api = vi.hoisted (() => ({
-  apiPut: vi.fn (),
+  apiPut:     vi.fn (),
+  isApiError: vi.fn (),
 }))
 
 const toastApi = vi.hoisted (() => ({
@@ -23,6 +24,11 @@ vi.mock ('@/components/users/InheritDialogue', () => ({
 }))
 
 describe ('SettingPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    api.isApiError.mockReturnValue (false)
+  })
+
   it ('shows loading when user is absent', () => {
     renderWithProviders (<SettingPage user={null} setUser={vi.fn ()}/>)
 
@@ -51,4 +57,28 @@ describe ('SettingPage', () => {
     expect (setUser).toHaveBeenCalled ()
     expect (toastApi.toast).toHaveBeenCalledWith ({ title: '設定を更新しました．' })
   })
+
+  it ('shows validation errors returned for the name field', async () => {
+    const user = buildUser ({ id: 11, name: 'old' })
+    api.isApiError.mockReturnValue (true)
+    api.apiPut.mockRejectedValueOnce ({
+      response: {
+        status: 422,
+        data:   {
+          type:        'validation_error',
+          message:     '入力内容を確認してください．',
+          errors:      { name: ['名前は必須です．'] },
+          base_errors: [],
+        },
+      },
+    })
+
+    renderWithProviders (<SettingPage user={user} setUser={vi.fn ()}/>)
+
+    fireEvent.change (screen.getByRole ('textbox'), { target: { value: '' } })
+    fireEvent.click (screen.getByRole ('button', { name: '更新' }))
+
+    expect (await screen.findByText ('名前は必須です．')).toBeInTheDocument ()
+    expect (screen.getByRole ('textbox')).toHaveAttribute ('aria-invalid', 'true')
+  })
 })
diff --git a/frontend/src/pages/users/SettingPage.tsx b/frontend/src/pages/users/SettingPage.tsx
index 6632e5f..a271da9 100644
--- a/frontend/src/pages/users/SettingPage.tsx
+++ b/frontend/src/pages/users/SettingPage.tsx
@@ -3,7 +3,9 @@ import type { FC } from 'react'
 import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
+import FieldError from '@/components/common/FieldError'
 import Form from '@/components/common/Form'
+import FormField from '@/components/common/FormField'
 import Label from '@/components/common/Label'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
@@ -13,22 +15,30 @@ import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiPut } from '@/lib/api'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 
 import type { User } from '@/types'
 
 type Props = { user:    User | null
 	       setUser: React.Dispatch<React.SetStateAction<User | null>> }
 
+type UserFormField = 'name'
+
 
 const SettingPage: FC<Props> = ({ user, setUser }) => {
   const [name, setName] = useState ('')
   const [userCodeVsbl, setUserCodeVsbl] = useState (false)
   const [inheritVsbl, setInheritVsbl] = useState (false)
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<UserFormField> ()
 
   const handleSubmit = async () => {
     if (!(user))
       return
 
+    clearValidationErrors ()
+
     const formData = new FormData
     formData.append ('name', name)
 
@@ -40,8 +50,9 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
       setUser (user => ({ ...user, ...data }))
       toast ({ title: '設定を更新しました．' })
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: 'しっぱい……' })
     }
   }
@@ -65,11 +76,16 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 
 	  {user ? (
 	      <>
+		<FieldError messages={baseErrors}/>
+
 		{/* 名前 */}
-		<div>
-		  <Label>表示名</Label>
+		<FormField label="表示名" messages={fieldErrors.name}>
+		  {({ describedBy, invalid }) => (
+		      <>
 		  <input type="text"
-			 className="w-full border rounded p-2"
+			 aria-describedby={describedBy}
+			 aria-invalid={invalid}
+			 className={inputClass (invalid)}
 			 value={name}
 			 placeholder="名もなきニジラー"
 			 onChange={ev => setName (ev.target.value)}/>
@@ -77,7 +93,8 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 		      <p className="mt-1 text-sm text-red-500">
 			名前が未設定のアカウントは 30 日間アクセスしないと削除されます!!!!
 		      </p>)}
-		</div>
+		      </>)}
+		</FormField>
 
 		{/* 送信 */}
 		<Button onClick={handleSubmit}
diff --git a/frontend/src/pages/wiki/WikiEditPage.tsx b/frontend/src/pages/wiki/WikiEditPage.tsx
index c524b39..dcbe12c 100644
--- a/frontend/src/pages/wiki/WikiEditPage.tsx
+++ b/frontend/src/pages/wiki/WikiEditPage.tsx
@@ -5,11 +5,16 @@ import { Helmet } from 'react-helmet-async'
 import MdEditor from 'react-markdown-editor-lite'
 import { useParams, useNavigate } from 'react-router-dom'
 
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiGet, apiPut } from '@/lib/api'
 import { wikiKeys } from '@/lib/queryKeys'
+import { canEditContent } from '@/lib/users'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 import Forbidden from '@/pages/Forbidden'
 
 import 'react-markdown-editor-lite/lib/index.css'
@@ -22,9 +27,11 @@ const mdParser = new MarkdownIt
 
 type Props = { user: User | null }
 
+type WikiFormField = 'title' | 'body'
+
 
 const WikiEditPage: FC<Props> = ({ user }) => {
-  const editable = ['admin', 'member'].some (r => user?.role === r)
+  const editable = canEditContent (user)
 
   const { id } = useParams ()
 
@@ -35,8 +42,12 @@ const WikiEditPage: FC<Props> = ({ user }) => {
   const [body, setBody] = useState ('')
   const [loading, setLoading] = useState (true)
   const [title, setTitle] = useState ('')
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<WikiFormField> ()
 
   const handleSubmit = async () => {
+    clearValidationErrors ()
+
     const formData = new FormData ()
     formData.append ('title', title)
     formData.append ('body', body)
@@ -51,8 +62,9 @@ const WikiEditPage: FC<Props> = ({ user }) => {
       toast ({ title: '投稿成功!' })
       navigate (`/wiki/${ title }`)
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '投稿失敗', description: '入力を確認してください。' })
     }
   }
@@ -83,24 +95,28 @@ const WikiEditPage: FC<Props> = ({ user }) => {
 
 	  {loading ? 'Loading...' : (
 	      <>
+		<FieldError messages={baseErrors}/>
+
 		{/* タイトル */}
 		{/* TODO: タグ補完 */}
-		<div>
-		  <label className="block font-semibold mb-1">タイトル</label>
-		  <input type="text"
-			 value={title}
-			 onChange={e => setTitle (e.target.value)}
-			 className="w-full border p-2 rounded"/>
-		</div>
+		<FormField label="タイトル" messages={fieldErrors.title}>
+		  {({ describedBy, invalid }) => (
+		      <input type="text"
+			     value={title}
+			     onChange={e => setTitle (e.target.value)}
+			     aria-describedby={describedBy}
+			     aria-invalid={invalid}
+			     className={inputClass (invalid)}/>)}
+		</FormField>
 
 		{/* 本文 */}
-		<div>
-		  <label className="block font-semibold mb-1">本文</label>
-		  <MdEditor value={body}
-			    style={{ height: '500px' }}
-			    renderHTML={text => mdParser.render (text)}
-			    onChange={({ text }) => setBody (text)}/>
-		</div>
+		<FormField label="本文" messages={fieldErrors.body}>
+		  {() => (
+		      <MdEditor value={body}
+				style={{ height: '500px' }}
+				renderHTML={text => mdParser.render (text)}
+				onChange={({ text }) => setBody (text)}/>)}
+		</FormField>
 
 		{/* 送信 */}
 		<button onClick={handleSubmit}
diff --git a/frontend/src/pages/wiki/WikiNewPage.tsx b/frontend/src/pages/wiki/WikiNewPage.tsx
index 4dfd960..fffb690 100644
--- a/frontend/src/pages/wiki/WikiNewPage.tsx
+++ b/frontend/src/pages/wiki/WikiNewPage.tsx
@@ -6,10 +6,15 @@ import { Helmet } from 'react-helmet-async'
 import MdEditor from 'react-markdown-editor-lite'
 import { useLocation, useNavigate } from 'react-router-dom'
 
+import FieldError from '@/components/common/FieldError'
+import FormField from '@/components/common/FormField'
 import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiPost } from '@/lib/api'
+import { canEditContent } from '@/lib/users'
+import { inputClass } from '@/lib/utils'
+import { useValidationErrors } from '@/lib/useValidationErrors'
 import Forbidden from '@/pages/Forbidden'
 
 import 'react-markdown-editor-lite/lib/index.css'
@@ -20,9 +25,11 @@ const mdParser = new MarkdownIt
 
 type Props = { user: User | null }
 
+type WikiFormField = 'title' | 'body'
+
 
 const WikiNewPage: FC<Props> = ({ user }) => {
-  const editable = ['admin', 'member'].some (r => user?.role === r)
+  const editable = canEditContent (user)
 
   const location = useLocation ()
   const navigate = useNavigate ()
@@ -32,8 +39,12 @@ const WikiNewPage: FC<Props> = ({ user }) => {
 
   const [title, setTitle] = useState (titleQuery)
   const [body, setBody] = useState ('')
+  const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
+      useValidationErrors<WikiFormField> ()
 
   const handleSubmit = async () => {
+    clearValidationErrors ()
+
     const formData = new FormData
     formData.append ('title', title)
     formData.append ('body', body)
@@ -45,8 +56,9 @@ const WikiNewPage: FC<Props> = ({ user }) => {
       toast ({ title: '投稿成功!' })
       navigate (`/wiki/${ data.title }`)
     }
-    catch
+    catch (e)
     {
+      applyValidationError (e)
       toast ({ title: '投稿失敗', description: '入力を確認してください。' })
     }
   }
@@ -61,25 +73,28 @@ const WikiNewPage: FC<Props> = ({ user }) => {
 	</Helmet>
 	<div className="max-w-xl mx-auto p-4 space-y-4">
 	  <h1 className="text-2xl font-bold mb-2">新規 Wiki ページ</h1>
+	  <FieldError messages={baseErrors}/>
 
 	  {/* タイトル */}
 	  {/* TODO: タグ補完 */}
-	  <div>
-	    <label className="block font-semibold mb-1">タイトル</label>
-	    <input type="text"
-		   value={title}
-		   onChange={e => setTitle (e.target.value)}
-		   className="w-full border p-2 rounded"/>
-	  </div>
+	  <FormField label="タイトル" messages={fieldErrors.title}>
+	    {({ describedBy, invalid }) => (
+		<input type="text"
+		       value={title}
+		       onChange={e => setTitle (e.target.value)}
+		       aria-describedby={describedBy}
+		       aria-invalid={invalid}
+		       className={inputClass (invalid)}/>)}
+	  </FormField>
 
 	  {/* 本文 */}
-	  <div>
-	    <label className="block font-semibold mb-1">本文</label>
-	    <MdEditor value={body}
-		      style={{ height: '500px' }}
-		      renderHTML={text => mdParser.render (text)}
-		      onChange={({ text }) => setBody (text)}/>
-	  </div>
+	  <FormField label="本文" messages={fieldErrors.body}>
+	    {() => (
+		<MdEditor value={body}
+			  style={{ height: '500px' }}
+			  renderHTML={text => mdParser.render (text)}
+			  onChange={({ text }) => setBody (text)}/>)}
+	  </FormField>
 
 	  {/* 送信 */}
 	  <button onClick={handleSubmit}
diff --git a/frontend/src/pages/wiki/WikiSearchPage.tsx b/frontend/src/pages/wiki/WikiSearchPage.tsx
index 6eb190e..07d651d 100644
--- a/frontend/src/pages/wiki/WikiSearchPage.tsx
+++ b/frontend/src/pages/wiki/WikiSearchPage.tsx
@@ -2,11 +2,12 @@ import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
 import PrefetchLink from '@/components/PrefetchLink'
+import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
 import { apiGet } from '@/lib/api'
-import { dateString } from '@/lib/utils'
+import { dateString, inputClass } from '@/lib/utils'
 
 import type { FormEvent , FC } from 'react'
 
@@ -43,22 +44,22 @@ const WikiSearchPage: FC = () => {
 	  <PageTitle>Wiki</PageTitle>
 	  <form onSubmit={handleSearch} className="space-y-2">
 	    {/* タイトル */}
-	    <div>
-	      <label>タイトル：</label><br />
-	      <input type="text"
-		     value={title}
-		     onChange={e => setTitle (e.target.value)}
-		     className="border p-1 w-full" />
-	    </div>
+	    <FormField label="タイトル">
+	      {({ invalid }) => (
+		  <input type="text"
+			 value={title}
+			 onChange={e => setTitle (e.target.value)}
+			 className={inputClass (invalid)}/>)}
+	    </FormField>
 
 	    {/* 内容 */}
-	    <div>
-	      <label>内容：</label><br />
-	      <input type="text"
-		     value={text}
-		     onChange={e => setText (e.target.value)}
-		     className="border p-1 w-full" />
-	    </div>
+	    <FormField label="内容">
+	      {({ invalid }) => (
+		  <input type="text"
+			 value={text}
+			 onChange={e => setText (e.target.value)}
+			 className={inputClass (invalid)}/>)}
+	    </FormField>
 
 	    {/* 検索 */}
 	    <div className="py-3">
diff --git a/frontend/src/types.ts b/frontend/src/types.ts
index e552f69..38f7508 100644
--- a/frontend/src/types.ts
+++ b/frontend/src/types.ts
@@ -52,6 +52,18 @@ export type FetchTagsParams = {
     limit:        number
     order:        FetchTagsOrder }
 
+export type FetchNicoTagsParams = {
+    name:       string
+    linkedTag:  string
+    linkStatus: 'all' | 'linked' | 'unlinked'
+    page:       number
+    limit:      number
+    order:      FetchNicoTagsOrder }
+
+export type FetchNicoTagsOrder = `${ FetchNicoTagsOrderField }:${ 'asc' | 'desc' }`
+
+export type FetchNicoTagsOrderField = 'name' | 'created_at' | 'updated_at'
+
 export type Material = {
     id:             number
     tag:            Tag
@@ -83,8 +95,9 @@ export type MenuVisibleItem = {
     subMenu:  SubMenuItem[] }
 
 export type NicoTag = Tag & {
-    category:   'nico'
-    linkedTags: Tag[] }
+    category:               'nico'
+    linkedTags:             Tag[]
+    recentPostTagCreatedAt: string | null }
 
 export type NiconicoMetadata = {
     currentTime:            number