diff --git a/backend/app/models/tag_name_sanitisation_rule.rb b/backend/app/models/tag_name_sanitisation_rule.rb
index bb13a53..37cde5f 100644
--- a/backend/app/models/tag_name_sanitisation_rule.rb
+++ b/backend/app/models/tag_name_sanitisation_rule.rb
@@ -10,8 +10,9 @@ class TagNameSanitisationRule < ApplicationRecord
   validate :source_pattern_must_be_regexp
 
   class << self
-    def sanitise(name) =
-        rules.reduce(name.dup) { |name, (pattern, replacement)| name.gsub(pattern, replacement) }
+    def sanitise(name)
+      rules.reduce(name.dup) { |name, (pattern, replacement)| name.gsub(pattern, replacement) }
+    end
 
     def apply!
       TagName.find_each do |tn|
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index 450921e..d5a4bdb 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -143,7 +143,8 @@ namespace :nico do
       desired_tag_ids = kept_tag_ids.to_a
 
       datum['tags'].each do |raw|
-        tag = ExternalTag.find_or_create_by!(platform: :nico, name: raw)
+        name = TagNameSanitisationRule.sanitise(raw)
+        tag = ExternalTag.find_or_create_by!(platform: :nico, name:)
 
         unless tag.nico_tag_versions.exists?
           NicoTagVersionRecorder.record!(external_tag: tag,
diff --git a/backend/spec/tasks/nico_sync_spec.rb b/backend/spec/tasks/nico_sync_spec.rb
index dbca0ee..696df06 100644
--- a/backend/spec/tasks/nico_sync_spec.rb
+++ b/backend/spec/tasks/nico_sync_spec.rb
@@ -15,6 +15,11 @@ RSpec.describe 'nico:sync' do
     NicoTagRelation.create!(nico_tag_id: nico_tag.id, tag_id: tag.id)
   end
 
+  def create_nico_sanitisation_rules!
+    TagNameSanitisationRule.create!(priority: 20, source_pattern: '\\?', replacement: '_')
+    TagNameSanitisationRule.create!(priority: 40, source_pattren: '_$', replacement: '')
+  end
+
   it '既存 post を見つけて、nico tag と linked tag を追加し、差分が出たら bot を付ける' do
     # 既存 post（正規表現で拾われるURL）
     post = Post.create!(
@@ -435,17 +440,24 @@ RSpec.describe 'nico:sync' do
     expect(post.tags.map(&:name)).not_to include('bot操作')
   end
 
-  it '外部タグが新規記載されたとき，その時点の連携タグを記載する' do
+  it 'サニタイズ後の既存外部タグを再利用し、その連携タグを記載する' do
     post = create_nico_sync_post!
 
+    create_nico_sanitisation_rules!
+
     external_tag = create_external_tag!('AAA')
     linked_tag = create_tag!('spec_linked', category: :general)
     link_nico_to_tag!(external_tag, linked_tag)
 
-    run_nico_sync_with_tags!(['AAA'])
+    expect {
+      run_nico_sync_with_tags!(['AAA?'])
+    }.not_to change(ExternalTag, :count)
 
-    expect(post.reload.external_tags).to include(external_tag)
+    post.reload
+
+    expect(post.external_tags).to contain_exactly(external_tag)
     expect(post.tags).to include(linked_tag)
+    expect(ExternalTag.exists?(platform: :nico, name: 'AAA?')).to be(false)
   end
 
   it '外部タグに差分がない場合，内外マッピングが変はっても連携タグを再評価しない' do
@@ -520,4 +532,51 @@ RSpec.describe 'nico:sync' do
     # 再記載時点の内外マッピングが新たに適用される．
     expect(post.tags).to include(new_linked_tag)
   end
+
+  it '外部タグだけの変更では bot を付けず、投稿履歴を記録する' do
+    post = create_nico_sync_post!
+    PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
+
+    create_nico_sanitisation_rules!
+
+    expect {
+      run_nico_sync_with_tags!(['AAA?', 'AAA?'])
+    }.to change(PostVersion, :count).by(1)
+        .and change(ExternalTag, :count).by(1)
+        .and change(PostExternalTag, :count).by(1)
+        .and change(NicoTagVersion, :count).by(1)
+        .and change(TagName, :count).by(0)
+
+    external = post.reload.external_tags.sole
+
+    expect(external).to have_attributes(
+        platform: 'nico',
+        name: 'AAA')
+    expect(ExternalTag.exists?(platform: :nico, name: 'AAA?')).to be(false)
+
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(post.post_versions.order(:version_no).last.tags_json)
+        .to include('external_tag_id' => external.id)
+
+    expect {
+      run_nico_sync_with_tags!(['AAA?'])
+    }.to change(PostVersion, :count).by(0)
+        .and change(NicoTagVersion, :count).by(0)
+
+    expect {
+      run_nico_sync_with_tags!([])
+    }.to change(PostVersion, :count).by(1)
+
+    expect(post.reload.external_tags).to be_empty
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+  end
+
+  it 'nico: prefix を含む従来の規則で外部タグ名をサニタイズする' do
+    post = create_nico_sync_post!
+
+    run_nico_sync_with_tags!(['foo:_bar'])
+
+    expect(post.reload.external_tags.sole)
+        .to have_attributes(platform: 'nico', name: 'foo:_bar')
+  end
 end
diff --git a/frontend/src/components/DraggableDroppableTagRow.tsx b/frontend/src/components/DraggableDroppableTagRow.tsx
index 49f9aed..7c74006 100644
--- a/frontend/src/components/DraggableDroppableTagRow.tsx
+++ b/frontend/src/components/DraggableDroppableTagRow.tsx
@@ -38,6 +38,7 @@ const DraggableDroppableTagRow: FC<Props> = ({
       { normal: { duration: .2, ease: 'easeOut' as const } },
   )
   const dndId = `tag-node:${ pathKey }`
+  const dndDisabled = tag.category === 'nico'
 
   const downPosRef = useRef<{ x: number; y: number } | null> (null)
   const armedRef = useRef (false)
@@ -62,7 +63,8 @@ const DraggableDroppableTagRow: FC<Props> = ({
   const { attributes,
 	  listeners,
 	  setNodeRef: setDragRef,
-	  transform } = useDraggable ({ id:    dndId,
+	  transform } = useDraggable ({ id:       dndId,
+					disabled: dndDisabled,
 					data:  { kind:   'tag',
 						 dndId,
 						 tagId:  tag.id,
@@ -70,8 +72,9 @@ const DraggableDroppableTagRow: FC<Props> = ({
 						 nestLevel } })
 
   const { setNodeRef: setDropRef, isOver: over } = useDroppable ({
-      id:   dndId,
-      data: { kind: 'tag', tagId: tag.id } })
+      id:       dndId,
+      disabled: dndDisabled,
+      data:     { kind: 'tag', tagId: tag.id } })
 
   const activeDragging = activeDndId === dndId
   const style: CSSProperties = { transform:   CSS.Translate.toString (transform),
diff --git a/frontend/src/components/TagDetailSidebar.tsx b/frontend/src/components/TagDetailSidebar.tsx
index 63fbfd2..1949ec5 100644
--- a/frontend/src/components/TagDetailSidebar.tsx
+++ b/frontend/src/components/TagDetailSidebar.tsx
@@ -150,16 +150,17 @@ const buildFlatTagByCategory = (
     byCategory: TagByCategory,
 ): TagByCategory => {
   const tagsTmp = { } as TagByCategory
-  const seen = new Set<number> ()
+  const seen = new Set<string> ()
 
   for (const category of CATEGORIES)
     tagsTmp[category] = []
 
   const visit = (tag: TagWithSections) => {
-    if (seen.has (tag.id))
+    const key = `${ tag.category }:${ tag.id }`
+    if (seen.has (key))
       return
 
-    seen.add (tag.id)
+    seen.add (key)
     tagsTmp[tag.category].push ({ ...tag, children: [] })
 
     for (const child of tag.children ?? [])
diff --git a/frontend/src/components/TagLink.tsx b/frontend/src/components/TagLink.tsx
index 455add6..04328c2 100644
--- a/frontend/src/components/TagLink.tsx
+++ b/frontend/src/components/TagLink.tsx
@@ -86,7 +86,7 @@ const TagLink: FC<Props> = ({ tag,
 	  className={cn (
 	      'inline-flex min-w-0 max-w-full flex-nowrap items-stretch align-baseline',
 	      'gap-x-1 md:items-baseline')}>
-	{(linkFlg && withWiki && isFullTag (tag)) && (
+	{(linkFlg && withWiki && isFullTag (tag) && tag.category !== 'nico') && (
 	    <span className={markerWrapClass}>
 	      {(tag.materialId != null || tag.hasWiki || tag.hasDeerjikists)
 	       ? (
diff --git a/frontend/src/lib/posts.ts b/frontend/src/lib/posts.ts
index cab0ed4..2b9822d 100644
--- a/frontend/src/lib/posts.ts
+++ b/frontend/src/lib/posts.ts
@@ -28,15 +28,18 @@ export const fetchPost = async (id: string): Promise<Post> => await apiGet (`/po
 
 
 export const fetchPostChanges = async (
-    { post, tag, page, limit }: {
-	post?:  string
-	tag?:   string
-	page:   number
-	limit:  number }): Promise<{
+    { post, tag, externalTag, page, limit }: {
+	post?:        string
+	tag?:         string
+	externalTag?: string
+	page:         number
+	limit:        number }): Promise<{
     versions: PostVersion[]
     count:    number }> =>
 	await apiGet ('/posts/versions', { params: { ...(post && { post }),
 						     ...(tag && { tag }),
+						     ...(externalTag && {
+						       external_tag: externalTag }),
 						     page, limit } })
 
 
diff --git a/frontend/src/lib/prefetchers.ts b/frontend/src/lib/prefetchers.ts
index 264a380..46254ea 100644
--- a/frontend/src/lib/prefetchers.ts
+++ b/frontend/src/lib/prefetchers.ts
@@ -128,6 +128,7 @@ const prefetchPostShow: Prefetcher = async (qc, url) => {
 const prefetchPostChanges: Prefetcher = async (qc, url) => {
   const id = url.searchParams.get ('id')
   const tag = url.searchParams.get ('tag')
+  const externalTag = url.searchParams.get ('external_tag')
   const page = Number (url.searchParams.get ('page') || 1)
   const limit = Number (url.searchParams.get ('limit') || 20)
 
@@ -141,9 +142,11 @@ const prefetchPostChanges: Prefetcher = async (qc, url) => {
   await qc.prefetchQuery ({
       queryKey: postsKeys.changes ({ ...(id && { id }),
 				     ...(tag && { tag }),
+				     ...(externalTag && { externalTag }),
 				     page, limit }),
       queryFn:  () => fetchPostChanges ({ ...(id && { id }),
 					  ...(tag && { tag }),
+					  ...(externalTag && { externalTag }),
 					  page, limit }) })
 }
 
diff --git a/frontend/src/lib/queryKeys.ts b/frontend/src/lib/queryKeys.ts
index 293f849..49c231f 100644
--- a/frontend/src/lib/queryKeys.ts
+++ b/frontend/src/lib/queryKeys.ts
@@ -11,7 +11,11 @@ export const postsKeys = {
     index:    (p: FetchPostsParams) => ['posts', 'index', p] as const,
     show:     (id: string) => ['posts', id] as const,
     related:  (id: string) => ['related', id] as const,
-    changes:  (p: { post?: string; tag?: string; page: number; limit: number }) =>
+    changes:  (p: { post?: string
+		    tag?: string
+		    externalTag?: string
+		    page: number
+		    limit: number }) =>
 		  ['posts', 'changes', p] as const }
 
 export const gekanatorKeys = {
diff --git a/frontend/src/pages/posts/PostHistoryPage.tsx b/frontend/src/pages/posts/PostHistoryPage.tsx
index 71aa3c8..70b517d 100644
--- a/frontend/src/pages/posts/PostHistoryPage.tsx
+++ b/frontend/src/pages/posts/PostHistoryPage.tsx
@@ -51,6 +51,7 @@ const PostHistoryPage: FC = () => {
   const query = new URLSearchParams (location.search)
   const id = query.get ('id')
   const tagId = query.get ('tag')
+  const externalTagId = query.get ('external_tag')
   const page = Number (query.get ('page') ?? 1)
   const limit = Number (query.get ('limit') ?? 20)
 
@@ -66,9 +67,12 @@ const PostHistoryPage: FC = () => {
   const { data, isLoading: loading } = useQuery ({
       queryKey: postsKeys.changes ({ ...(id && { post: id }),
 				     ...(tagId && { tag: tagId }),
+				     ...(externalTagId && { externalTag: externalTagId }),
 				     page, limit }),
       queryFn:  () => fetchPostChanges ({ ...(id && { post: id }),
 					  ...(tagId && { tag: tagId }),
+					  ...(externalTagId && {
+					    externalTag: externalTagId }),
 					  page, limit }) })
   const changes = data?.versions ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
diff --git a/frontend/src/pages/tags/TagListPage.tsx b/frontend/src/pages/tags/TagListPage.tsx
index 568c5d6..8dde00d 100644
--- a/frontend/src/pages/tags/TagListPage.tsx
+++ b/frontend/src/pages/tags/TagListPage.tsx
@@ -406,11 +406,15 @@ const TagListPage: FC = () => {
 
 		  <tbody>
 		    {results.map (row => (
-			<tr key={row.id} className="even:bg-gray-100 dark:even:bg-gray-700">
+			<tr
+			    key={`${ row.category }:${ row.id }`}
+			    className="even:bg-gray-100 dark:even:bg-gray-700">
 			  <td className="p-2">
 			    <TagLink
 				tag={row}
-				to={`/tags/${ encodeURIComponent (row.id) }`}
+				to={row.category === 'nico'
+				    ? `/tags/nico?name=${ encodeURIComponent (row.name) }`
+				    : `/tags/${ encodeURIComponent (row.id) }`}
 				withCount={false}/>
 			  </td>
 			  <td className="p-2 text-right">{row.postCount}</td>
@@ -429,7 +433,10 @@ const TagListPage: FC = () => {
 			  <td className="p-2">{dateString (row.createdAt)}</td>
 			  <td className="p-2">{dateString (row.updatedAt)}</td>
 			  <td className="p-2">
-			    <PrefetchLink to={`/posts/changes?tag=${ row.id }`}>
+			    <PrefetchLink
+				to={row.category === 'nico'
+				    ? `/posts/changes?external_tag=${ row.id }`
+				    : `/posts/changes?tag=${ row.id }`}>
 			      耕作履歴
 			    </PrefetchLink>
 			  </td>