diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index 6dd565b..24af551 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -17,7 +17,12 @@ class Post < ApplicationRecord
            foreign_key: :target_post_id
   has_one_attached :thumbnail
 
+  before_validation :normalise_url
+
+  validates :url, presence: true, uniqueness: true
+
   validate :validate_original_created_range
+  validate :url_must_be_http_url
 
   def as_json options = { }
     super(options).merge({ thumbnail: thumbnail.attached? ?
@@ -69,4 +74,33 @@ class Post < ApplicationRecord
       errors.add :original_created_before, 'オリジナルの作成日時の順番がをかしぃです．'
     end
   end
+
+  def url_must_be_http_url
+    begin
+      u = URI.parse(url)
+    rescue URI::InvalidURIError
+      errors.add(:url, 'URL が不正です．')
+      return
+    end
+
+    if !(u in URI::HTTP) || u.host.blank?
+      errors.add(:url, 'URL が不正です．')
+      return
+    end
+  end
+
+  def normalise_url
+    return if url.blank?
+
+    self.url = url.strip
+
+    u = URI.parse(url)
+    return unless u in URI::HTTP
+
+    u.host = u.host.downcase if u.host
+    u.path = u.path.sub(/\/\Z/, '') if u.path.present?
+    self.url = u.to_s
+  rescue URI::InvalidURIError
+    ;
+  end
 end
diff --git a/backend/config/locales/en.yml b/backend/config/locales/en.yml
index 6c349ae..ff6ebed 100644
--- a/backend/config/locales/en.yml
+++ b/backend/config/locales/en.yml
@@ -28,4 +28,8 @@
 #       enabled: "ON"
 
 en:
-  hello: "Hello world"
+  activerecord:
+    errors:
+      messages:
+        record_invalid: "Validation failed: %{errors}"
+        taken: 'イキスギ!'
diff --git a/backend/db/migrate/20260118144400_add_unique_index_to_url_in_posts.rb b/backend/db/migrate/20260118144400_add_unique_index_to_url_in_posts.rb
new file mode 100644
index 0000000..de807c3
--- /dev/null
+++ b/backend/db/migrate/20260118144400_add_unique_index_to_url_in_posts.rb
@@ -0,0 +1,6 @@
+class AddUniqueIndexToUrlInPosts < ActiveRecord::Migration[7.1]
+  def change
+    change_column :posts, :url, :string, limit: 768
+    add_index :posts, :url, unique: true, name: 'index_posts_on_url'
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index b8312fb..a67d0dd 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_01_12_111800) do
+ActiveRecord::Schema[8.0].define(version: 2026_01_18_144400) 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
@@ -85,7 +85,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_12_111800) do
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "title"
-    t.string "url", limit: 2000, null: false
+    t.string "url", limit: 768, null: false
     t.string "thumbnail_base", limit: 2000
     t.bigint "parent_id"
     t.bigint "uploaded_user_id"
@@ -95,6 +95,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_12_111800) do
     t.datetime "updated_at", null: false
     t.index ["parent_id"], name: "index_posts_on_parent_id"
     t.index ["uploaded_user_id"], name: "index_posts_on_uploaded_user_id"
+    t.index ["url"], name: "index_posts_on_url", unique: true
   end
 
   create_table "settings", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index 4e0cf11..817eec0 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -33,14 +33,14 @@ RSpec.describe 'Posts API', type: :request do
 
     let!(:hit_post) do
       Post.create!(uploaded_user: user, title: "hello spec world",
-                   url: 'https://example.com/spec').tap do |p|
+                   url: 'https://example.com/spec2').tap do |p|
         PostTag.create!(post: p, tag:)
       end
     end
 
     let!(:miss_post) do
       Post.create!(uploaded_user: user, title: "unrelated title",
-                   url: 'https://example.com/spec2').tap do |p|
+                   url: 'https://example.com/spec3').tap do |p|
         PostTag.create!(post: p, tag: tag2)
       end
     end
@@ -158,6 +158,36 @@ RSpec.describe 'Posts API', type: :request do
       expect(json['tags']).to be_an(Array)
       expect(json['tags'][0]).to have_key('name')
     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
+        }
+
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+    end
+
+    context 'when url is invalid' do
+      it 'returns 422' do
+        sign_in_as(member)
+
+        post '/posts', params: {
+          title: 'new post',
+          url: 'ぼざクリタグ広場',
+          tags: 'spec_tag', # 既存タグ名を投げる
+          thumbnail: dummy_upload
+        }
+
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+    end
   end
 
   describe 'PUT /posts/:id' do
diff --git a/backend/spec/requests/wiki_spec.rb b/backend/spec/requests/wiki_spec.rb
index 6b5fe1f..4541501 100644
--- a/backend/spec/requests/wiki_spec.rb
+++ b/backend/spec/requests/wiki_spec.rb
@@ -8,7 +8,9 @@ RSpec.describe 'Wiki API', type: :request do
 
   let!(:tn) { TagName.create!(name: 'spec_wiki_title') }
   let!(:page) do
-    WikiPage.create!(tag_name: tn, created_user: user, updated_user: user)
+    WikiPage.create!(tag_name: tn, created_user: user, updated_user: user).tap do |p|
+      Wiki::Commit.content!(page: p, body: 'init', created_user: user, message: 'init')
+    end
   end
 
   describe 'GET /wiki' do