diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index eaf51da..1fd85e6 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -160,9 +160,15 @@ class Tag < ApplicationRecord
         end
 
         tag_name = st.tag_name
+        nico_flg = st.nico?
         st.destroy!
         tag_name.reload
-        tag_name.update!(canonical: target_tag.tag_name)
+
+        if nico_flg
+          tag_name.destroy!
+        else
+          tag_name.update!(canonical: target_tag.tag_name)
+        end
       end
 
       # 投稿件数を再集計
diff --git a/backend/app/models/tag_name.rb b/backend/app/models/tag_name.rb
index 225343d..5b39a06 100644
--- a/backend/app/models/tag_name.rb
+++ b/backend/app/models/tag_name.rb
@@ -1,4 +1,6 @@
 class TagName < ApplicationRecord
+  before_validation :sanitise_name
+
   has_one :tag
   has_one :wiki_page
 
@@ -22,6 +24,10 @@ class TagName < ApplicationRecord
 
   private
 
+  def sanitise_name
+    self.name = TagNameSanitisationRule.sanitise(name)
+  end
+
   def canonical_must_be_canonical
     if canonical&.canonical_id?
       errors.add :canonical, 'canonical は実体を示す必要があります．'
diff --git a/backend/app/models/tag_name_sanitisation_rule.rb b/backend/app/models/tag_name_sanitisation_rule.rb
new file mode 100644
index 0000000..d0c8ea1
--- /dev/null
+++ b/backend/app/models/tag_name_sanitisation_rule.rb
@@ -0,0 +1,55 @@
+class TagNameSanitisationRule < ApplicationRecord
+  include Discard::Model
+
+  self.primary_key = :priority
+
+  default_scope -> { kept }
+
+  validates :source_pattern, presence: true, uniqueness: true
+
+  validate :source_pattern_must_be_regexp
+
+  class << self
+    def sanitise(name) =
+        rules.reduce(name.dup) { |name, (pattern, replacement)| name.gsub(pattern, replacement) }
+
+    def apply!
+      TagName.find_each do |tn|
+        name = sanitise(tn.name)
+        next if name == tn.name
+
+        TagName.transaction do
+          existing_tn = TagName.find_by(name:)
+          if existing_tn
+            existing_tn = existing_tn.canonical || existing_tn
+            next if existing_tn.id == tn.id
+
+            if existing_tn.tag
+              Tag.merge_tags!(existing_tn.tag, tn.tag) if tn.tag
+            elsif tn.tag
+              tn.tag.update_columns(tag_name_id: existing_tn.id, updated_at: Time.current)
+            end
+            tn.destroy!
+
+            next
+          end
+
+          # TagName 側の自動サニタイズを回避
+          tn.update_columns(name:, updated_at: Time.current)
+        end
+      end
+    end
+
+    private
+
+    def rules = kept.order(:priority).map { |r| [Regexp.new(r.source_pattern), r.replacement] }
+  end
+
+  private
+
+  def source_pattern_must_be_regexp
+    Regexp.new(source_pattern)
+  rescue RegexpError
+    errors.add :source_pattern, '変な正規表現だね〜（笑）'
+  end
+end
diff --git a/backend/db/migrate/20260309123200_create_tag_name_sanitisation_rules.rb b/backend/db/migrate/20260309123200_create_tag_name_sanitisation_rules.rb
new file mode 100644
index 0000000..e187ce9
--- /dev/null
+++ b/backend/db/migrate/20260309123200_create_tag_name_sanitisation_rules.rb
@@ -0,0 +1,31 @@
+class CreateTagNameSanitisationRules < ActiveRecord::Migration[8.0]
+  def up
+    create_table :tag_name_sanitisation_rules, id: :integer, primary_key: :priority do |t|
+      t.string :source_pattern, null: false
+      t.string :replacement, null: false
+      t.timestamps
+      t.datetime :discarded_at
+      t.index :source_pattern, unique: true
+      t.index :discarded_at
+    end
+
+    now = ActiveRecord::Base.connection.quote(Time.current)
+    execute <<~SQL
+      INSERT INTO
+        tag_name_sanitisation_rules(priority, source_pattern, replacement, created_at, updated_at)
+      VALUES
+        (10, '\\\\*',           '_',         #{ now }, #{ now })
+      , (20, '\\\\?',           '_',         #{ now }, #{ now })
+      , (25, '\\\\/',           '_',         #{ now }, #{ now })
+      , (30, '_+',              '_',         #{ now }, #{ now })
+      , (40, '_$',              '',          #{ now }, #{ now })
+      , (45, '^([^:]+\\\\:)?_', '\\\\1',     #{ now }, #{ now })
+      , (50, '^([^:]+\\\\:)?$', '\\\\1null', #{ now }, #{ now })
+      ;
+    SQL
+  end
+
+  def down
+    drop_table :tag_name_sanitisation_rules
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index 8077fac..58a7158 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_03_03_122700) do
+ActiveRecord::Schema[8.0].define(version: 2026_03_09_123200) 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
@@ -127,6 +127,16 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_03_122700) do
     t.index ["tag_id"], name: "index_tag_implications_on_tag_id"
   end
 
+  create_table "tag_name_sanitisation_rules", primary_key: "priority", id: :integer, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "source_pattern", null: false
+    t.string "replacement", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
+    t.index ["discarded_at"], name: "index_tag_name_sanitisation_rules_on_discarded_at"
+    t.index ["source_pattern"], name: "index_tag_name_sanitisation_rules_on_source_pattern", unique: true
+  end
+
   create_table "tag_names", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.bigint "canonical_id"
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index 6ee9c26..09be474 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -108,7 +108,7 @@ namespace :nico do
       desired_non_nico_tag_ids = []
 
       datum['tags'].each do |raw|
-        name = "nico:#{ raw }"
+        name = TagNameSanitisationRule.sanitise("nico:#{ raw }")
         tag = Tag.find_or_create_by_tag_name!(name, category: :nico)
         desired_nico_tag_based_ids << tag.id
 
diff --git a/backend/spec/models/tag_name_sanitisation_rule_spec.rb b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
new file mode 100644
index 0000000..9a95c3b
--- /dev/null
+++ b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
@@ -0,0 +1,63 @@
+require 'rails_helper'
+
+RSpec.describe TagNameSanitisationRule, type: :model do
+  describe '.sanitise' do
+    before do
+      described_class.create!(priority: 10, source_pattern: '_', replacement: '')
+      described_class.create!(priority: 20, source_pattern: 'ABC', replacement: 'abc')
+    end
+
+    it 'applies rules in priority order' do
+      expect(described_class.sanitise('A_B_C')).to eq('abc')
+    end
+
+    it 'does not fail when a rule does not match' do
+      expect { described_class.sanitise('xyz') }.not_to raise_error
+      expect(described_class.sanitise('xyz')).to eq('xyz')
+    end
+  end
+
+  describe 'validations' do
+    it 'is invalid when source_pattern is not a valid regexp' do
+      rule = described_class.new(priority: 10, source_pattern: '[', replacement: '')
+      expect(rule).to be_invalid
+      expect(rule.errors[:source_pattern]).to be_present
+    end
+  end
+
+  describe '.apply!' do
+    before do
+      described_class.create!(priority: 10, source_pattern: '_', replacement: '')
+    end
+
+    context 'when no conflicting tag_name exists' do
+      let!(:tag_name) { TagName.create!(name: 'foo_bar') }
+
+      it 'renames the tag_name' do
+        described_class.apply!
+        expect(tag_name.reload.name).to eq('foobar')
+      end
+    end
+
+    context 'when a conflicting canonical tag_name exists' do
+      let!(:existing) { TagName.create!(name: 'foobar') }
+      let!(:source)   { TagName.create!(name: 'foo_bar') }
+
+      it 'deletes the source tag_name' do
+        described_class.apply!
+        expect(TagName.exists?(source.id)).to be(false)
+        expect(existing.reload.name).to eq('foobar')
+      end
+    end
+
+    context 'when the source tag_name has a tag and the existing one has no tag' do
+      let!(:existing) { TagName.create!(name: 'foobar') }
+      let!(:source_tag) { create(:tag, name: 'foo_bar', category: :general) }
+
+      it 'moves the tag to the existing tag_name' do
+        described_class.apply!
+        expect(source_tag.reload.tag_name_id).to eq(existing.id)
+      end
+    end
+  end
+end
diff --git a/backend/spec/models/tag_name_spec.rb b/backend/spec/models/tag_name_spec.rb
new file mode 100644
index 0000000..b89583c
--- /dev/null
+++ b/backend/spec/models/tag_name_spec.rb
@@ -0,0 +1,28 @@
+require 'rails_helper'
+
+RSpec.describe TagName, type: :model do
+  describe 'before_validation :sanitise_name' do
+    before do
+      TagNameSanitisationRule.create!(priority: 10, source_pattern: '_', replacement: '')
+    end
+
+    it 'sanitises name on create' do
+      tag_name = TagName.create!(name: 'foo_bar')
+      expect(tag_name.reload.name).to eq('foobar')
+    end
+
+    it 'sanitises name on update' do
+      tag_name = TagName.create!(name: 'foobar')
+      tag_name.update!(name: 'foo_bar')
+      expect(tag_name.reload.name).to eq('foobar')
+    end
+
+    it 'becomes invalid when sanitised name conflicts with another tag_name' do
+      TagName.create!(name: 'foobar')
+      tag_name = TagName.new(name: 'foo_bar')
+
+      expect(tag_name).to be_invalid
+      expect(tag_name.errors[:name]).to be_present
+    end
+  end
+end
diff --git a/backend/spec/models/tag_spec.rb b/backend/spec/models/tag_spec.rb
index 389f1a1..b63710a 100644
--- a/backend/spec/models/tag_spec.rb
+++ b/backend/spec/models/tag_spec.rb
@@ -2,10 +2,13 @@ require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
   describe '.merge_tags!' do
-    let!(:target_tag) { create(:tag) }
-    let!(:source_tag) { create(:tag) }
+    let!(:target_tag) { create(:tag, category: :general) }
+    let!(:source_tag) { create(:tag, category: :general) }
+    let!(:source_tag_name) { source_tag.tag_name }
 
-    let!(:post_record) { Post.create!(url: 'https://example.com/posts/1', title: 'test post') }
+    let!(:post_record) do
+      Post.create!(url: 'https://example.com/posts/1', title: 'test post')
+    end
 
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
@@ -15,7 +18,8 @@ RSpec.describe Tag, type: :model do
 
         expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
         expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
@@ -31,8 +35,10 @@ RSpec.describe Tag, type: :model do
 
         expect(active.count).to eq(1)
         expect(discarded_source.discarded_at).to be_present
+        expect(discarded_source.tag_id).to eq(source_tag.id)
         expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
@@ -45,6 +51,7 @@ RSpec.describe Tag, type: :model do
         expect(Tag.exists?(target_tag.id)).to be(true)
         expect(Tag.exists?(source_tag.id)).to be(false)
         expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
+        expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
@@ -52,9 +59,10 @@ RSpec.describe Tag, type: :model do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
       let!(:wiki_page) do
         WikiPage.create!(
-            tag_name: source_tag.tag_name,
-            created_user: create_admin_user!,
-            updated_user: create_admin_user!)
+          tag_name: source_tag_name,
+          created_user: create_admin_user!,
+          updated_user: create_admin_user!
+        )
       end
 
       it 'rolls back the transaction' do
@@ -64,7 +72,20 @@ RSpec.describe Tag, type: :model do
 
         expect(Tag.exists?(source_tag.id)).to be(true)
         expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
-        expect(source_tag.tag_name.reload.canonical_id).to be_nil
+        expect(source_tag_name.reload.canonical_id).to be_nil
+      end
+    end
+
+    context 'when merging a nico source tag' do
+      let!(:target_tag) { create(:tag, category: :nico, name: 'nico:foo') }
+      let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
+      let!(:source_tag_name_id) { source_tag.tag_name_id }
+
+      it 'deletes the source tag_name instead of aliasing it' do
+        described_class.merge_tags!(target_tag, [source_tag])
+
+        expect(Tag.exists?(source_tag.id)).to be(false)
+        expect(TagName.exists?(source_tag_name_id)).to be(false)
       end
     end
   end