diff --git a/backend/Gemfile b/backend/Gemfile
index bb5460b..303b937 100644
--- a/backend/Gemfile
+++ b/backend/Gemfile
@@ -63,3 +63,5 @@ gem 'diff-lcs'
 gem 'dotenv-rails'
 
 gem 'whenever', require: false
+
+gem 'discard'
diff --git a/backend/Gemfile.lock b/backend/Gemfile.lock
index 8494a53..2c08f92 100644
--- a/backend/Gemfile.lock
+++ b/backend/Gemfile.lock
@@ -90,6 +90,8 @@ GEM
     crass (1.0.6)
     date (3.4.1)
     diff-lcs (1.6.2)
+    discard (1.4.0)
+      activerecord (>= 4.2, < 9.0)
     dotenv (3.1.8)
     dotenv-rails (3.1.8)
       dotenv (= 3.1.8)
@@ -420,6 +422,7 @@ DEPENDENCIES
   bootsnap
   brakeman
   diff-lcs
+  discard
   dotenv-rails
   gollum
   image_processing (~> 1.14)
diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 6877c70..bd64ea9 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -1,7 +1,3 @@
-require 'open-uri'
-require 'nokogiri'
-
-
 class PostsController < ApplicationController
   # GET /posts
   def index
@@ -77,7 +73,7 @@ class PostsController < ApplicationController
     post.thumbnail.attach(thumbnail)
     if post.save
       post.resized_thumbnail!
-      post.tags = Tag.normalise_tags(tag_names)
+      sync_post_tags!(post, Tag.normalise_tags(tag_names))
       render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
              status: :created
     else
@@ -110,8 +106,10 @@ class PostsController < ApplicationController
     original_created_before = params[:original_created_before]
 
     post = Post.find(params[:id].to_i)
-    tags = post.tags.where(category: 'nico').to_a + Tag.normalise_tags(tag_names)
-    if post.update(title:, tags:, original_created_from:, original_created_before:)
+    if post.update(title:, original_created_from:, original_created_before:)
+      sync_post_tags!(post,
+                      (post.tags.where(category: 'nico').to_a +
+                       Tag.normalise_tags(tag_names)))
       render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
              status: :ok
     else
@@ -128,7 +126,11 @@ class PostsController < ApplicationController
   def filtered_posts
     tag_names = params[:tags]&.split(' ')
     match_type = params[:match]
-    tag_names.present? ? filter_posts_by_tags(tag_names, match_type) : Post.all
+    if tag_names.present?
+      filter_posts_by_tags(tag_names, match_type)
+    else
+      Post.all
+    end
   end
 
   def filter_posts_by_tags tag_names, match_type
@@ -142,4 +144,28 @@ class PostsController < ApplicationController
     end
     posts.distinct
   end
+
+  def sync_post_tags! post, desired_tags
+    desired_tags.each do |t|
+      t.save! if t.new_record?
+    end
+
+    desired_ids = desired_tags.map(&:id).to_set
+    current_ids = post.tags.pluck(:id).to_set
+
+    to_add = desired_ids - current_ids
+    to_remove = current_ids - desired_ids
+
+    Tag.where(id: to_add).find_each do |tag|
+      begin
+        PostTag.create!(post:, tag:, created_user: current_user)
+      rescue ActiveRecord::RecordNotUnique
+        ;
+      end
+    end
+
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+      pt.discard_by!(current_user)
+    end
+  end
 end
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index f32f753..0e37223 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -1,11 +1,13 @@
-require 'mini_magick'
-
-
 class Post < ApplicationRecord
+  require 'mini_magick'
+
   belongs_to :parent, class_name: 'Post', optional: true, foreign_key: 'parent_id'
   belongs_to :uploaded_user, class_name: 'User', optional: true
-  has_many :post_tags, dependent: :destroy
-  has_many :tags, through: :post_tags
+
+  has_many :post_tags, dependent: :delete_all, inverse_of: :post
+  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
+  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
+  has_many :tags, through: :active_post_tags
   has_many :user_post_views, dependent: :destroy
   has_many :post_similarities_as_post,
            class_name: 'PostSimilarity',
diff --git a/backend/app/models/post_tag.rb b/backend/app/models/post_tag.rb
index 9dbd756..91a739d 100644
--- a/backend/app/models/post_tag.rb
+++ b/backend/app/models/post_tag.rb
@@ -1,7 +1,25 @@
 class PostTag < ApplicationRecord
+  include Discard::Model
+
   belongs_to :post
   belongs_to :tag, counter_cache: :post_count
+  belongs_to :created_user, class_name: 'User', optional: true
+  belongs_to :deleted_user, class_name: 'User', optional: true
 
   validates :post_id, presence: true
   validates :tag_id, presence: true
+  validates :post_id, uniqueness: {
+      scope: :tag_id,
+      conditions: -> { where(discarded_at: nil) } }
+
+  def discard_by! deleted_user
+    return self if discarded?
+
+    transaction do
+      update!(discarded_at: Time.current, deleted_user:)
+      Tag.where(id: tag_id).update_all('post_count = GREATEST(post_count - 1, 0)')
+    end
+
+    self
+  end
 end
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 137afff..4ed034c 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -1,6 +1,8 @@
 class Tag < ApplicationRecord
-  has_many :post_tags, dependent: :destroy
-  has_many :posts, through: :post_tags
+  has_many :post_tags, dependent: :delete_all, inverse_of: :tag
+  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', :inverse_of: :tag
+  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
+  has_many :posts, through: :active_post_tags
   has_many :tag_aliases, dependent: :destroy
 
   has_many :nico_tag_relations, foreign_key: :nico_tag_id, dependent: :destroy
@@ -35,13 +37,13 @@ class Tag < ApplicationRecord
       'meta:' => 'meta' }.freeze
 
   def self.tagme
-    @tagme ||= Tag.find_or_initialize_by(name: 'タグ希望') do |tag|
+    @tagme ||= Tag.find_or_create_by!(name: 'タグ希望') do |tag|
       tag.category = 'meta'
     end
   end
 
   def self.bot
-    @bot ||= Tag.find_or_initialize_by(name: 'bot操作') do |tag|
+    @bot ||= Tag.find_or_create_by!(name: 'bot操作') do |tag|
       tag.category = 'meta'
     end
   end
diff --git a/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb b/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb
new file mode 100644
index 0000000..11634ad
--- /dev/null
+++ b/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb
@@ -0,0 +1,18 @@
+class AddDiscardToPostTags < ActiveRecord::Migration[8.0]
+  def change
+    add_column :post_tags, :discarded_at, :datetime
+    add_index :post_tags, :discarded_at
+
+    add_column :post_tags, :is_active, :boolean,
+               as: 'discarded_at IS NULL', stored: true
+
+    add_column :post_tags, :active_unique_key, :string,
+               as: "CASE WHEN discarded_at IS NULL THEN CONCAT(post_id, ':', tag_id) ELSE NULL END",
+               stored: true
+
+    add_index :post_tags, :active_unique_key, unique: true, name: 'idx_post_tags_active_unique'
+
+    add_index :post_tags, [:post_id, :discarded_at]
+    add_index :post_tags, [:tag_id, :discarded_at]
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index 4d5cfd6..f55b323 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: 2025_09_09_075500) do
+ActiveRecord::Schema[8.0].define(version: 2025_10_11_200300) 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
@@ -70,9 +70,16 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_09_075500) do
     t.bigint "deleted_user_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
+    t.virtual "is_active", type: :boolean, as: "(`discarded_at` is null)", stored: true
+    t.virtual "active_unique_key", type: :string, as: "(case when (`discarded_at` is null) then concat(`post_id`,_utf8mb4':',`tag_id`) else NULL end)", stored: true
+    t.index ["active_unique_key"], name: "idx_post_tags_active_unique", unique: true
     t.index ["created_user_id"], name: "index_post_tags_on_created_user_id"
     t.index ["deleted_user_id"], name: "index_post_tags_on_deleted_user_id"
+    t.index ["discarded_at"], name: "index_post_tags_on_discarded_at"
+    t.index ["post_id", "discarded_at"], name: "index_post_tags_on_post_id_and_discarded_at"
     t.index ["post_id"], name: "index_post_tags_on_post_id"
+    t.index ["tag_id", "discarded_at"], name: "index_post_tags_on_tag_id_and_discarded_at"
     t.index ["tag_id"], name: "index_post_tags_on_tag_id"
   end
 
@@ -107,6 +114,15 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_09_075500) do
     t.index ["tag_id"], name: "index_tag_aliases_on_tag_id"
   end
 
+  create_table "tag_implications", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_id", null: false
+    t.bigint "parent_tag_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["parent_tag_id"], name: "index_tag_implications_on_parent_tag_id"
+    t.index ["tag_id"], name: "index_tag_implications_on_tag_id"
+  end
+
   create_table "tag_similarities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "target_tag_id", null: false
@@ -176,6 +192,8 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_09_075500) do
   add_foreign_key "posts", "users", column: "uploaded_user_id"
   add_foreign_key "settings", "users"
   add_foreign_key "tag_aliases", "tags"
+  add_foreign_key "tag_implications", "tags"
+  add_foreign_key "tag_implications", "tags", column: "parent_tag_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
   add_foreign_key "user_ips", "ip_addresses"
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index a8601c5..00e309f 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -5,12 +5,30 @@ namespace :nico do
     require 'open-uri'
     require 'nokogiri'
 
-    fetch_thumbnail = -> url {
+    fetch_thumbnail = -> url do
       html = URI.open(url, read_timeout: 60, 'User-Agent' => 'Mozilla/5.0').read
       doc = Nokogiri::HTML(html)
 
       doc.at('meta[name="thumbnail"]')&.[]('content').presence
-    }
+    end
+
+    def sync_post_tags! post, desired_tag_ids
+      desired_ids = desired_tag_ids.compact.to_set
+      current_ids = post.tags.pluck(:id).to_set
+
+      to_add = desired_ids - current_ids
+      to_remove = current_ids - desired_ids
+
+      Tag.where(id: to_add.to_a).find_each do |tag|
+        begin
+          PostTag.create!(post:, tag:)
+        rescue ActiveRecord::RecordNotUnique
+          ;
+        end
+      end
+
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each(&:discard!)
+    end
 
     mysql_user = ENV['MYSQL_USER']
     mysql_pass = ENV['MYSQL_PASS']
@@ -19,43 +37,53 @@ namespace :nico do
         { 'MYSQL_USER' => mysql_user, 'MYSQL_PASS' => mysql_pass },
         'python3', "#{ nizika_nico_path }/get_videos.py")
 
-    if status.success?
-      data = JSON.parse(stdout)
-      data.each do |datum|
-        post = Post.where('url LIKE ?', '%nicovideo.jp%').find { |post|
-          post.url =~ %r{#{ Regexp.escape(datum['code']) }(?!\d)}
-        }
-        unless post
-          title = datum['title']
-          url = "https://www.nicovideo.jp/watch/#{ datum['code'] }"
-          thumbnail_base = fetch_thumbnail.(url) || '' rescue ''
-          post = Post.new(title:, url:, thumbnail_base:, uploaded_user: nil)
-          if thumbnail_base.present?
-            post.thumbnail.attach(
-                io:           URI.open(thumbnail_base),
-                filename:     File.basename(URI.parse(thumbnail_base).path),
-                content_type: 'image/jpeg')
-          end
-          post.save!
-          post.resized_thumbnail!
+    abort unless status.success?
+
+    data = JSON.parse(stdout)
+    data.each do |datum|
+      post = Post.where('url LIKE ?', '%nicovideo.jp%').find { |post|
+        post.url =~ %r{#{ Regexp.escape(datum['code']) }(?!\d)}
+      }
+      unless post
+        title = datum['title']
+        url = "https://www.nicovideo.jp/watch/#{ datum['code'] }"
+        thumbnail_base = fetch_thumbnail.(url) || '' rescue ''
+        post = Post.new(title:, url:, thumbnail_base:, uploaded_user: nil)
+        if thumbnail_base.present?
+          post.thumbnail.attach(
+              io:           URI.open(thumbnail_base),
+              filename:     File.basename(URI.parse(thumbnail_base).path),
+              content_type: 'image/jpeg')
         end
+        post.save!
+        post.resized_thumbnail!
+      end
+
+      kept_tags = post.tags.reload
+      kept_non_nico_ids = kept_tags.where.not(category: 'nico').pluck(:id).to_set
 
-        current_tags = post.tags.where(category: 'nico').pluck(:name).sort
-        new_tags = datum['tags'].map { |tag| "nico:#{ tag }" }.sort
-        if current_tags != new_tags
-          post.tags.destroy(post.tags.where(name: current_tags))
-          tags_to_add = []
-          new_tags.each do |name|
-            tag = Tag.find_or_initialize_by(name:) do |t|
-              t.category = 'nico'
-            end
-            tags_to_add.concat([tag] + tag.linked_tags)
-          end
-          tags_to_add << Tag.tagme if post.tags.size < 20
-          tags_to_add << Tag.bot
-          post.tags = (post.tags + tags_to_add).uniq
+      desired_nico_ids = []
+      datum['tags'].each do |raw|
+        name = "nico:#{ raw }"
+        tag = Tag.find_or_initialize_by(name:) do |t|
+          t.category = 'nico'
         end
+        tag.save! if tag.new_record?
+        desired_nico_ids << tag.id
+        desired_nico_ids.concat(tag.linked_tags.pluck(:id))
       end
+      desired_nico_ids.uniq!
+
+      desired_extra_ids = []
+      desired_extra_ids << Tag.tagme.id if kept_tags.size < 10
+      desired_extra_ids << Tag.bot.id
+      desired_extra_ids.compact!
+      desired_extra_ids.uniq!
+
+      desired_all_ids = kept_non_nico_ids.to_a + desired_nico_ids + desired_extra_ids
+      desired_all_ids.uniq!
+
+      sync_post_tags!(post, desired_all_ids)
     end
   end
 end