diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 363b926..272db8b 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -173,15 +173,41 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    base_version_no = parse_base_version_no
+    force = truthy_param?(params[:force])
+
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
     original_created_from = params[:original_created_from]
     original_created_before = params[:original_created_before]
     parent_post_ids = parse_parent_post_ids
 
-    post = Post.find(params[:id].to_i)
+    post = nil
+    conflict_json = nil
 
     ApplicationRecord.transaction do
+      post = Post.find(params[:id].to_i)
+
+      base_version = post.post_versions.find_by!(version_no: base_version_no)
+
+      base_snapshot = post_snapshot_from_version(base_version)
+      current_snapshot = post_snapshot_from_record(post)
+      incoming_snapshot = post_incoming_snapshot(post,
+                                                 title:,
+                                                 original_created_from:,
+                                                 original_created_before:,
+                                                 tag_names:,
+                                                 parent_post_ids:)
+
+      if !(force) && post.version_no != base_version_no
+        conflict_json = post_conflict_json(post:,
+                                           base_version_no:,
+                                           base_snapshot:,
+                                           current_snapshot:,
+                                           incoming_snapshot:)
+        raise ActiveRecord::Rollback
+      end
+
       PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
 
       post.update!(title:, original_created_from:, original_created_before:)
@@ -198,8 +224,10 @@ class PostsController < ApplicationController
       PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
     end
 
+    return render json: conflict_json, status: :conflict if conflict_json
+
     post.reload
-    json = post.as_json
+    json = PostRepr.base(post, current_user)
     json['tags'] = build_tag_tree_for(post.tags)
     render json:, status: :ok
   rescue Tag::NicoTagNormalisationError
@@ -404,4 +432,178 @@ class PostsController < ApplicationController
       PostImplication.create_or_find_by!(post_id: post.id, parent_post_id:)
     end
   end
+
+  def parse_base_version_no
+    version_no = Integer(params[:base_version_no], exception: false)
+    raise ArgumentError, 'base_version_no は必須です．' unless version_no&.positive?
+
+    version_no
+  end
+
+  def truthy_param?(value) = ActiveModel::Type::Boolean.new.cast(value)
+
+  def post_snapshot_from_version version
+    { title: version.title,
+      original_created_from: snapshot_time(version.original_created_from),
+      original_created_before: snapshot_time(version.original_created_before),
+      tag_names: version.tags.to_s.split.sort,
+      parent_post_ids: snapshot_parent_post_ids_from_version(version) }
+  end
+
+  def post_snapshot_form_record post
+    { title: post.title,
+      original_created_from: snapshot_time(post.original_created_from),
+      original_created_before: snapshot_time(post.original_created_before),
+      tag_names: post.tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name'),
+      parent_post_ids: post.parent_posts.order(:id).pluck(:id) }
+  end
+
+  def post_incoming_snapshot post, title:, original_created_from:, original_created_before:,
+                                   tag_names:, parent_post_ids:
+    { title:
+      original_created_from: snapshot_time(original_created_from),
+      original_created_before: snapshot_time(original_created_before),
+      tag_names: incoming_tag_names_for_snapshot(post, tag_names),
+      parent_post_ids: parent_post_ids.sort }
+  end
+
+  def snapshot_parent_post_ids_from_version version
+    if version.respond_to?(:parent_post_ids)
+      version.parent_post_ids.to_s.split.map { |id| id.to_i }.sort
+    elsif version.respond_to?(:parent_id) && version.parent_id
+      [version.parent_id]
+    else
+      []
+    end
+  end
+
+  def snapshot_time value
+    return nil if value.blank?
+
+    value = Time.zone.parse(value.to_s) if value in String
+    value&.in_time_zone&.iso8601(6)
+  rescue ArgumentError, TypeError
+    value.to_s
+  end
+
+  def incoming_tag_names_for_snapshot post, raw_tag_names
+    manual_names = normalised_manual_tag_names_for_snapshot(raw_tag_names)
+    nico_names = post.tags.nico.joins(:tag_name).pluck('tag_names.name')
+
+    existing_tags =
+        Tag
+        .joins(:tag_name)
+        .where(tag_names: { name: manual_names + nico_names })
+        .to_a
+
+    expanded_names = Tag.expand_parent_tags(existing_tags).map(&:name)
+
+    (manual_names + nico_names + expanded_names).uniq.sort
+  end
+
+  def normalised_manual_tag_names_for_snapshot raw_tag_names
+    if raw_tag_names.any? { |name| name.downcase.start_with?('nico:') }
+      raise Tag::NicoTagNormalisationError
+    end
+
+    pairs = raw_tag_names.map do |raw_name|
+      prefix, category =
+          Tag::CATEGORY_PREFIXES.find { |p, _| raw_name.downcase.start_with?(p) } || ['', nil]
+
+      name = TagName.canonicalise(raw_name.sub(/\A#{ Regexp.escape(prefix) }/i, '')).first
+
+      [name, category]
+    end
+
+    names = pairs.map(&:first)
+
+    has_deerjikist = pairs.any? do |name, category|
+      category == :deerjikist ||
+          Tag.joins(:tag_name).where(category: :deerjikist, tag_names: { name: }).exists?
+    end
+
+    names << Tag.no_deerjikist.name unless has_deerjikist
+
+    names.uniq.sort
+  end
+
+  def post_conflict_json post:, base_version_no:, base_snapshot:,
+                         current_snapshot:, incoming_snapshot:
+    changes = post_snapshot_changes(base_snapshot, current_snapshot, incoming_snapshot)
+    conflicts = changes.select { |change| change[:conflict] }
+
+    { error: 'conflict',
+      message: '競合が発生しました．',
+      post_id: post.id,
+      base_version_no:,
+      current_version_no: post.version_no,
+      base: base_snapshot,
+      current: current_snapshot,
+      mine: incoming_snapshot,
+      changes:,
+      conflicts:,
+      mergeable: conflicts.empty? }
+  end
+
+  def post_snapshot_changes base_snapshot, current_snapshot, incoming_snapshot
+    [scalar_snapshot_change(:title, 'タイトル',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     scalar_snapshot_change(:original_created_from, '元コンテンツ作成日時（開始）',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     scalar_snapshot_change(:original_created_before, '元コンテンツ作成日時（終了）',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     set_snapshot_change(:tag_names, 'タグ',
+                         base_snapshot, current_snapshot, incoming_snapshot),
+     set_snapshot_change(:parent_post_ids, '親投稿',
+                         base_snapshot, current_snapshot, incoming_snapshot)].compact
+  end
+
+  def scalar_snapshot_change field, label, base_snapshot, current_snapshot, incoming_snapshot
+    base = base_snapshot[field]
+    current = current_snapshot[field]
+    mine = incoming_snapshot[field]
+
+    return nil if current == base && mine == base
+
+    { field:, label:, base:, current:, mine:,
+      changed_by_current: current != base,
+      changed_by_me: mine != base,
+      conflict: scalar_snapshot_conflict?(base, current, mine) }
+  end
+
+  def scalar_snapshot_conflict? base, current, mine
+    current != base && mine != base && current != mine
+  end
+
+  def set_snapshot_change field, label, base_snapshot, current_snapshot, incoming_snapshot
+    base = base_snapshot[field].to_a
+    current = current_snapshot[field].to_a
+    mine = incoming_snapshot[field].to_a
+
+    added_by_current = current - base
+    removed_by_current = base - current
+    added_by_me = mine - base
+    removed_by_me = base - mine
+
+    if (added_by_current.empty? &&
+        removed_by_current.empty? &&
+        added_by_me.empty? &&
+        removed_by_me.empty?)
+      return nil
+    end
+
+    { field:, label:, base:, current:, mine:, added_by_current:, removed_by_current:,
+      added_by_me:, removed_by_me:,
+      changed_by_current: added_by_current.present? || removed_by_current.present?,
+      changed_by_me: added_by_me.present? || removed_by_me.present?,
+      conflict: set_snapshot_conflict?(added_by_current:,
+                                       removed_by_current:,
+                                       added_by_me:,
+                                       removed_by_me:) }
+  end
+
+  def set_snapshot_conflict? added_by_current:, removed_by_current:,
+                             added_by_me:, removed_by_me:
+    (added_by_current & removed_by_me).present? || (removed_by_current & added_by_me).present?
+  end
 end
diff --git a/backend/app/services/version_recorder.rb b/backend/app/services/version_recorder.rb
index e705ec3..fe7c368 100644
--- a/backend/app/services/version_recorder.rb
+++ b/backend/app/services/version_recorder.rb
@@ -16,19 +16,20 @@ class VersionRecorder
       @record = record_class.unscoped.lock.find(@record.id)
       latest = latest_version
 
-      if !(latest) && @event_type != 'create'
-        raise "#{ version_class.name } first event must be create"
-      end
+      validate_version_sequence! latest
+
+      attrs = snapshot_attributes
 
-      if @event_type == 'create' && latest
-        raise "#{ version_class.name } create event already exists"
+      if @event_type == 'update' && latest && same_snapshot?(latest, attrs)
+        return latest
       end
 
-      attrs = snapshot_attributes
+      version = version_class.create!(
+          base_attributes(latest).merge(record_key => @record).merge(attrs))
 
-      return latest if @event_type == 'update' && latest && same_snapshot?(latest, attrs)
+      update_record_version_no! version.version_no
 
-      version_class.create!(base_attributes(latest).merge(record_key => @record).merge(attrs))
+      version
     end
   end
 
@@ -45,7 +46,31 @@ class VersionRecorder
       created_by_user: @created_by_user }
   end
 
-  def same_snapshot?(version, attrs) = attrs.all? { |k, v| version.public_send(k) == v }
+  def update_record_version_no! version_no
+    @record.update_columns version_no: version_no
+    @record.version_no = version_no
+  end
+
+  def validate_version_sequence! latest
+    if !(latest) && @event_type != 'create'
+      raise "#{ version_class.name } first event must be create"
+    end
+
+    if @event_type == 'create' && latest
+      raise "#{ version_class.name } create event already exists"
+    end
+
+    return unless latest
+
+    if @record.version_no != latest.version_no
+      raise ("#{ record_class.name }##{ @record.id } version_no is #{ @record.version_no }, " +
+             "but latest #{ version_class.name } version_no is #{ latest.version_no }")
+    end
+  end
+
+  def same_snapshot? version, attrs
+    attrs.all? { |k, v| version.public_send(k) == v }
+  end
 
   def validate_event_type!
     return if EVENT_TYPES.include?(@event_type)
diff --git a/backend/db/migrate/20260507124000_add_version_no_to_posts.rb b/backend/db/migrate/20260507124000_add_version_no_to_posts.rb
new file mode 100644
index 0000000..7161a23
--- /dev/null
+++ b/backend/db/migrate/20260507124000_add_version_no_to_posts.rb
@@ -0,0 +1,27 @@
+class AddVersionNoToPosts < ActiveRecord::Migration[8.0]
+  def up
+    add_column :posts, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        posts
+      SET
+        version_no = (
+          SELECT
+            MAX(version_no)
+          FROM
+            post_versions
+          WHERE
+            post_id = posts.id)
+    SQL
+
+    change_column_null :posts, :version_no, false
+
+    add_check_constraint :posts, 'version_no > 0', name: 'chk_posts_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :posts, name: 'chk_posts_version_no_positive'
+    remove_column :posts, :version_no
+  end
+end
diff --git a/backend/db/migrate/20260507211600_add_version_no_to_tags.rb b/backend/db/migrate/20260507211600_add_version_no_to_tags.rb
new file mode 100644
index 0000000..2a4ef30
--- /dev/null
+++ b/backend/db/migrate/20260507211600_add_version_no_to_tags.rb
@@ -0,0 +1,37 @@
+class AddVersionNoToTags < ActiveRecord::Migration[8.0]
+  def up
+    add_column :tags, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        tags
+      SET
+        version_no = (
+          CASE category
+          WHEN 'nico' THEN
+            (SELECT
+               MAX(version_no)
+             FROM
+               nico_tag_versions
+             WHERE
+               tag_id = tags.id)
+          ELSE
+            (SELECT
+               MAX(version_no)
+             FROM
+               tag_versions
+             WHERE
+               tag_id = tags.id)
+          END)
+    SQL
+
+    change_column_null :tags, :version_no, false
+
+    add_check_constraint :tags, 'version_no > 0', name: 'chk_tags_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :tags, name: 'chk_tags_version_no_positive'
+    remove_column :tags, :version_no
+  end
+end
diff --git a/backend/db/migrate/20260507213300_add_version_no_to_wiki_pages.rb b/backend/db/migrate/20260507213300_add_version_no_to_wiki_pages.rb
new file mode 100644
index 0000000..be34918
--- /dev/null
+++ b/backend/db/migrate/20260507213300_add_version_no_to_wiki_pages.rb
@@ -0,0 +1,27 @@
+class AddVersionNoToWikiPages < ActiveRecord::Migration[8.0]
+  def up
+    add_column :wiki_pages, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        wiki_pages
+      SET
+        version_no = (
+          SELECT
+            MAX(version_no)
+          FROM
+            wiki_versions
+          WHERE
+            wiki_page_id = wiki_pages.id)
+    SQL
+
+    change_column_null :wiki_pages, :version_no, false
+
+    add_check_constraint :wiki_pages, 'version_no > 0', name: 'chk_wiki_pages_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :wiki_pages, name: 'chk_wiki_pages_version_no_positive'
+    remove_column :wiki_pages, :version_no
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index 042d227..94edb82 100644
--- a/backend/db/schema.rb
+++ b/backend/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
+ActiveRecord::Schema[8.0].define(version: 2026_05_07_213300) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -186,8 +186,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "original_created_from"
     t.datetime "original_created_before"
     t.datetime "updated_at", null: false
+    t.integer "version_no", null: false
     t.index ["uploaded_user_id"], name: "index_posts_on_uploaded_user_id"
     t.index ["url"], name: "index_posts_on_url", unique: true
+    t.check_constraint "`version_no` > 0", name: "chk_posts_version_no_positive"
   end
 
   create_table "settings", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -262,8 +264,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "updated_at", null: false
     t.integer "post_count", default: 0, null: false
     t.datetime "discarded_at"
+    t.integer "version_no", null: false
     t.index ["discarded_at"], name: "index_tags_on_discarded_at"
     t.index ["tag_name_id"], name: "index_tags_on_tag_name_id", unique: true
+    t.check_constraint "`version_no` > 0", name: "chk_tags_version_no_positive"
   end
 
   create_table "theatre_comments", primary_key: ["theatre_id", "no"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -369,10 +373,12 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "updated_at", null: false
     t.datetime "discarded_at"
     t.integer "next_asset_no", default: 1, null: false
+    t.integer "version_no", null: false
     t.index ["created_user_id"], name: "index_wiki_pages_on_created_user_id"
     t.index ["discarded_at"], name: "index_wiki_pages_on_discarded_at"
     t.index ["tag_name_id"], name: "index_wiki_pages_on_tag_name_id", unique: true
     t.index ["updated_user_id"], name: "index_wiki_pages_on_updated_user_id"
+    t.check_constraint "`version_no` > 0", name: "chk_wiki_pages_version_no_positive"
   end
 
   create_table "wiki_revision_lines", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|