diff --git a/backend/app/models/nico_tag_relation.rb b/backend/app/models/nico_tag_relation.rb
index 239047c..7156dec 100644
--- a/backend/app/models/nico_tag_relation.rb
+++ b/backend/app/models/nico_tag_relation.rb
@@ -10,7 +10,7 @@ class NicoTagRelation < ApplicationRecord
   private
 
   def nico_tag_must_be_nico
-    if nico_tag && nico_tag.category != :nico
+    if nico_tag && nico_tag.category != 'nico'
       errors.add :nico_tag_id, 'タグのカテゴリがニコニコである必要があります．'
     end
   end
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index 605f627..ecc2f17 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -1,6 +1,6 @@
 class Post < ApplicationRecord
   belongs_to :parent, class_name: 'Post', optional: true, foreign_key: 'parent_id'
-  belongs_to :user, foreign_key: ''
+  belongs_to :uploaded_user, class_name: 'User', optional: true
   has_many :post_tags, dependent: :destroy
   has_many :tags, through: :post_tags
   has_many :user_post_views, dependent: :destroy
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 5ed4968..3b3c6a3 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -15,5 +15,15 @@ class Tag < ApplicationRecord
   validates :name, presence: true, length: { maximum: 255 }
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
+  validate :nico_tag_name_must_start_with_nico
+
   scope :nico_tags, -> { where category: :nico }
+
+  private
+
+  def nico_tag_name_must_start_with_nico
+    if category == 'nico' && name&.[](0, 5) != 'nico:'
+      errors.add :name, 'ニコニコ・タグの命名規則に反してゐます．'
+    end
+  end
 end