diff --git a/app/models/post.rb b/app/models/post.rb
index 2bd1abb01..5394e19e0 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -6,6 +6,7 @@ class Post < ActiveRecord::Base
   class DisapprovalError < Exception ; end
   class RevertError < Exception ; end
   class SearchError < Exception ; end
+  class DeletionError < Exception ; end
 
   before_validation :initialize_uploader, :on => :create
   before_validation :merge_old_changes
@@ -65,6 +66,12 @@ class Post < ActiveRecord::Base
 
     module ClassMethods
       def delete_files(post_id, file_path, large_file_path, preview_file_path)
+        post = Post.find(post_id)
+
+        if post.file_path == file_path || post.large_file_path == large_file_path || post.preview_file_path == preview_file_path
+          raise DeletionError.new("Files still in use; skipping deletion.")
+        end
+
         # the large file and the preview don't necessarily exist. if so errors will be ignored.
         FileUtils.rm_f(file_path)
         FileUtils.rm_f(large_file_path)
diff --git a/test/unit/post_replacement_test.rb b/test/unit/post_replacement_test.rb
index 293de1943..23c48bd7e 100644
--- a/test/unit/post_replacement_test.rb
+++ b/test/unit/post_replacement_test.rb
@@ -170,5 +170,25 @@ class PostReplacementTest < ActiveSupport::TestCase
         assert_equal([{"file"=>"000000.jpg", "delay"=>125}, {"file"=>"000001.jpg", "delay"=>125}], @post.pixiv_ugoira_frame_data.data)
       end
     end
+
+    context "a post that is replaced to another file then replaced back to the original file" do
+      should "not delete the original files" do
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+
+        assert(File.exists?(@post.file_path))
+        assert(File.exists?(@post.preview_file_path))
+        assert(File.exists?(@post.large_file_path))
+
+        Timecop.travel(Time.now + PostReplacement::DELETION_GRACE_PERIOD + 1.day) do
+          Delayed::Worker.new.work_off
+        end
+
+        assert(File.exists?(@post.file_path))
+        assert(File.exists?(@post.preview_file_path))
+        assert(File.exists?(@post.large_file_path))
+      end
+    end
   end
 end