diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 648d42e..dde9f15 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -81,8 +81,6 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.member?
 
-    # TODO: URL が正規のものがチェック，不正ならエラー
-    # TODO: URL は必須にする（タイトルは省略可）．
     # TODO: サイトに応じて thumbnail_base 設定
     title = params[:title].presence
     url = params[:url]
@@ -105,6 +103,8 @@ class PostsController < ApplicationController
     else
       render json: { errors: post.errors.full_messages }, status: :unprocessable_entity
     end
+  rescue Tag::NicoTagNormalisationError
+    head :bad_request
   end
 
   def viewed
@@ -142,6 +142,8 @@ class PostsController < ApplicationController
     else
       render json: post.errors, status: :unprocessable_entity
     end
+  rescue Tag::NicoTagNormalisationError
+    head :bad_request
   end
 
   def changes
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 17546b5..a1372c7 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -1,4 +1,8 @@
 class Tag < ApplicationRecord
+  class NicoTagNormalisationError < StandardError
+    ;
+  end
+
   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'
@@ -65,6 +69,8 @@ class Tag < ApplicationRecord
 
   def self.normalise_tags tag_names, with_tagme: true
     tags = tag_names.map do |name|
+      raise NicoTagNormalisationError if name.start_with?('nico:')
+
       pf, cat = CATEGORY_PREFIXES.find { |p, _| name.start_with?(p) } || ['', nil]
       name.delete_prefix!(pf)
       find_or_create_by_tag_name!(name, category: (cat || 'general')).tap do |tag|
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index eb7ce9f..07523b1 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -167,16 +167,34 @@ RSpec.describe 'Posts API', type: :request do
       expect(json['tags'][0]).to have_key('name')
     end
 
+    context "when nico tag already exists in tags" do
+      before do
+        Tag.find_or_create_by!(tag_name: TagName.find_or_create_by!(name: 'nico:nico_tag'),
+                               category: 'nico')
+      end
+
+      it 'return 400' do
+        sign_in_as(member)
+
+        post '/posts', params: {
+            title: 'new post',
+            url: 'https://example.com/nico_tag',
+            tags: 'nico:nico_tag',
+            thumbnail: dummy_upload }
+
+        expect(response).to have_http_status(:bad_request)
+      end
+    end
+
     context 'when url is blank' do
       it 'returns 422' do
         sign_in_as(member)
 
         post '/posts', params: {
-          title: 'new post',
-          url: '   ',
-          tags: 'spec_tag', # 既存タグ名を投げる
-          thumbnail: dummy_upload
-        }
+            title: 'new post',
+            url: '   ',
+            tags: 'spec_tag', # 既存タグ名を投げる
+            thumbnail: dummy_upload }
 
         expect(response).to have_http_status(:unprocessable_entity)
       end
@@ -233,6 +251,23 @@ RSpec.describe 'Posts API', type: :request do
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
     end
+
+    context "when nico tag already exists in tags" do
+      before do
+        Tag.find_or_create_by!(tag_name: TagName.find_or_create_by!(name: 'nico:nico_tag'),
+                               category: 'nico')
+      end
+
+      it 'return 400' do
+        sign_in_as(member)
+
+        put "/posts/#{ post_record.id }", params: {
+            title: 'updated title',
+            tags: 'nico:nico_tag' }
+
+        expect(response).to have_http_status(:bad_request)
+      end
+    end
   end
 
   describe 'GET /posts/random' do