diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index e689891..dae8994 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -265,14 +265,17 @@ class TagsController < ApplicationController
   end
 
   def show
-    tag = Tag.joins(:tag_name)
-             .includes(:tag_name, :materials, tag_name: :wiki_page)
-             .find_by(id: params[:id])
-    if tag
-      render json: TagRepr.base(tag)
-    else
-      head :not_found
-    end
+    tag =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .find_by(id: params[:id])
+    return render json: TagRepr.base(tag) if tag
+
+    external_tag = ExternalTag.find_by(id: params[:id])
+    return render json: ExternalTagRepr.base(external_tag) if external_tag
+
+    head :not_found
   end
 
   def show_by_name
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index d5a4bdb..db386a3 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -143,7 +143,7 @@ namespace :nico do
       desired_tag_ids = kept_tag_ids.to_a
 
       datum['tags'].each do |raw|
-        name = TagNameSanitisationRule.sanitise(raw)
+        name = TagNameSanitisationRule.sanitise("nico:#{ raw }").delete_prefix('nico:')
         tag = ExternalTag.find_or_create_by!(platform: :nico, name:)
 
         unless tag.nico_tag_versions.exists?
diff --git a/backend/spec/requests/tags_spec.rb b/backend/spec/requests/tags_spec.rb
index c9c36fe..b7232da 100644
--- a/backend/spec/requests/tags_spec.rb
+++ b/backend/spec/requests/tags_spec.rb
@@ -379,6 +379,37 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:not_found)
       end
     end
+
+    it 'falls back to ExternalTag when no internal Tag has the id' do
+      external = create(
+          :external_tag,
+          id: Tag.maximum(:id).to_i + 10_000,
+          name: 'legacy_id_lookup')
+
+      get "/tags/#{ external.id }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => external.id,
+          'name' => 'nico:legacy_id_lookup',
+          'category' => 'nico')
+    end
+
+    it 'prefers Tag when Tag and ExternalTag have the same id' do
+      internal = create(:tag, name: 'internal_collision')
+      create(
+          :external_tag,
+          id: internal.id,
+          name: 'external_collision')
+
+      get "/tags/#{ internal.id }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => internal.id,
+          'name' => 'internal_collision')
+      expect(json.fetch('category')).not_to eq('nico')
+    end
   end
 
   describe 'GET /tags/autocomplete' do
diff --git a/backend/spec/tasks/nico_sync_spec.rb b/backend/spec/tasks/nico_sync_spec.rb
index 3788917..b3c053f 100644
--- a/backend/spec/tasks/nico_sync_spec.rb
+++ b/backend/spec/tasks/nico_sync_spec.rb
@@ -16,8 +16,17 @@ RSpec.describe 'nico:sync' do
   end
 
   def create_nico_sanitisation_rules!
-    TagNameSanitisationRule.create!(priority: 20, source_pattern: '\\?', replacement: '_')
-    TagNameSanitisationRule.create!(priority: 40, source_pattern: '_$', replacement: '')
+    TagNameSanitisationRule.create!(priority: 20,
+                                    source_pattern: '\\?',
+                                    replacement: '_')
+
+    TagNameSanitisationRule.create!(priority: 40,
+                                    source_pattern: '_$',
+                                    replacement: '')
+
+    TagNameSanitisationRule.create!(priority: 45,
+                                    source_pattern: '^([^:]+\\:)?_',
+                                    replacement: '\\1')
   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
@@ -573,6 +582,7 @@ RSpec.describe 'nico:sync' do
 
   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!
+    create_nico_sanitisation_rules!
 
     run_nico_sync_with_tags!(['foo:_bar'])