diff --git a/app/models/smile.rb b/app/models/smile.rb
index e94c1bd4..566c3bcc 100644
--- a/app/models/smile.rb
+++ b/app/models/smile.rb
@@ -1,7 +1,7 @@
 class Smile < ActiveRecord::Base
   belongs_to :user
   belongs_to :answer
-  validates :user_id, presence: true
+  validates :user_id, presence: true, uniqueness: { scope: :answer_id, message: "already smiled answer" }
   validates :answer_id, presence: true
 
   after_create do