diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 5658ebb..f76e83a 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -216,7 +216,8 @@ class PostsController < ApplicationController
 
     preflight = PostCreatePreflight.new(
         attributes: post_create_attributes,
-        thumbnail: params[:thumbnail]).run
+        thumbnail: params[:thumbnail],
+        host: request.base_url).run
     return render json: dry_run_json(preflight) if bool?(:dry)
     if preflight[:existing_post_id].present?
       post = Post.new(url: preflight[:url])
@@ -273,7 +274,11 @@ class PostsController < ApplicationController
     return head :payload_too_large if request.content_length.to_i > MAX_BULK_REQUEST_BYTES
     posts = parse_bulk_posts_manifest
     thumbnails = parse_bulk_thumbnails(posts.length)
-    result = PostBulkCreator.new(actor: current_user, posts:, thumbnails:).run
+    result = PostBulkCreator.new(
+        actor: current_user,
+        posts:,
+        thumbnails:,
+        host: request.base_url).run
     render json: result
   rescue JSON::ParserError
     render_bad_request 'posts manifest の JSON が不正です．'
@@ -627,7 +632,7 @@ class PostsController < ApplicationController
     return nil if post_id.blank?
 
     post = Post.with_attached_thumbnail.find_by(id: post_id)
-    PostCompactRepr.base(post)
+    PostCompactRepr.base(post, host: request.base_url)
   end
 
   def dry_run_json preflight
diff --git a/backend/app/representations/post_compact_repr.rb b/backend/app/representations/post_compact_repr.rb
index eca81aa..94fcdc7 100644
--- a/backend/app/representations/post_compact_repr.rb
+++ b/backend/app/representations/post_compact_repr.rb
@@ -4,11 +4,11 @@
 module PostCompactRepr
   module_function
 
-  def base post
+  def base post, host: nil
     return nil if post.nil?
 
     PostRepr
-        .common(post)
+        .common(post, host:)
         .slice(
             'id',
             'title',
diff --git a/backend/app/representations/post_repr.rb b/backend/app/representations/post_repr.rb
index d113263..2ea60fe 100644
--- a/backend/app/representations/post_repr.rb
+++ b/backend/app/representations/post_repr.rb
@@ -17,8 +17,8 @@ module PostRepr
 
   module_function
 
-  def base post, current_user = nil
-    json = common(post)
+  def base post, current_user = nil, host: nil
+    json = common(post, host:)
     json['tags'] = tag_json(post)
     json['uploaded_user'] = post.uploaded_user && UserRepr.base(post.uploaded_user)
     json['viewed'] = current_user ? current_user.viewed?(post) : false
@@ -26,31 +26,31 @@ module PostRepr
   end
 
   def detail post, current_user = nil, parent_posts: [], child_posts: [],
-             sibling_posts: { }, related: []
-    base(post, current_user).merge(
-        'parent_posts' => cards(parent_posts),
-        'child_posts' => cards(child_posts),
+             sibling_posts: { }, related: [], host: nil
+    base(post, current_user, host:).merge(
+        'parent_posts' => cards(parent_posts, host:),
+        'child_posts' => cards(child_posts, host:),
         'sibling_posts' => sibling_posts.transform_keys(&:to_s).transform_values { |posts|
-          cards(posts)
+          cards(posts, host:)
         },
-        'related' => cards(related))
+        'related' => cards(related, host:))
   end
 
-  def card post
-    common(post).merge('parent_posts' => [], 'child_posts' => [])
+  def card post, host: nil
+    common(post, host:).merge('parent_posts' => [], 'child_posts' => [])
   end
 
-  def cards posts
-    posts.map { |post| card(post) }
+  def cards posts, host: nil
+    posts.map { |post| card(post, host:) }
   end
 
-  def many posts, current_user = nil
-    posts.map { |p| base(p, current_user) }
+  def many posts, current_user = nil, host: nil
+    posts.map { |p| base(p, current_user, host:) }
   end
 
-  def common post
+  def common post, host: nil
     BASE_FIELDS.to_h { |field| [field.to_s, post.public_send(field)] }
-               .merge('thumbnail' => thumbnail_url(post))
+               .merge('thumbnail' => thumbnail_url(post, host:))
   end
 
   def tag_json post
@@ -65,10 +65,13 @@ module PostRepr
     }
   end
 
-  def thumbnail_url post
+  def thumbnail_url post, host: nil
     return nil unless post.thumbnail.attached?
 
-    Rails.application.routes.url_helpers.rails_blob_url(post.thumbnail, only_path: false)
+    Rails.application.routes.url_helpers.rails_storage_proxy_url(
+        post.thumbnail,
+        only_path: false,
+        host:)
   rescue ActionController::UrlGenerationError, ArgumentError, URI::InvalidURIError => e
     Rails.logger.warn(
         "PostRepr.thumbnail_url failed post_id=#{post.id} " \
diff --git a/backend/app/services/post_bulk_creator.rb b/backend/app/services/post_bulk_creator.rb
index 8bb6eca..1e75e24 100644
--- a/backend/app/services/post_bulk_creator.rb
+++ b/backend/app/services/post_bulk_creator.rb
@@ -1,8 +1,9 @@
 class PostBulkCreator
-  def initialize actor:, posts:, thumbnails:
+  def initialize actor:, posts:, thumbnails:, host: nil
     @actor_id = actor.id
     @posts = posts
     @thumbnails = thumbnails
+    @host = host
   end
 
   def run
@@ -64,7 +65,8 @@ class PostBulkCreator
     preflight =
         PostCreatePreflight.new(
             attributes: attributes,
-            thumbnail: thumbnail_for(index, attributes)).run
+            thumbnail: thumbnail_for(index, attributes),
+            host: @host).run
     if preflight[:existing_post_id].present?
       return {
           status: 'skipped',
@@ -195,6 +197,6 @@ class PostBulkCreator
   end
 
   def compact_existing_post post
-    PostCompactRepr.base(post)
+    PostCompactRepr.base(post, host: @host)
   end
 end
diff --git a/backend/app/services/post_create_preflight.rb b/backend/app/services/post_create_preflight.rb
index c633b47..04d3492 100644
--- a/backend/app/services/post_create_preflight.rb
+++ b/backend/app/services/post_create_preflight.rb
@@ -9,9 +9,10 @@ class PostCreatePreflight
     end
   end
 
-  def initialize attributes:, thumbnail: nil
+  def initialize attributes:, thumbnail: nil, host: nil
     @attributes = attributes.symbolize_keys
     @thumbnail = thumbnail
+    @host = host
   end
 
   def run
@@ -124,7 +125,7 @@ class PostCreatePreflight
     return nil if post_id.blank?
 
     post = Post.with_attached_thumbnail.find_by(id: post_id)
-    PostCompactRepr.base(post)
+    PostCompactRepr.base(post, host: @host)
   end
 
   def final_field_warnings field_warnings