diff --git a/ChangeLog b/ChangeLog
index 5626e54286..a1429505ec 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Wed Aug  6 19:09:27 2014  Akinori MUSHA  <knu@iDaemons.org>
+
+	* lib/set.rb (Set): Implement Set#clone by splitting
+	  initialize_copy into initialize_dup and initialize_clone.
+	  Submitted by yui-knk. [Fixes GH-661]
+	  https://github.com/ruby/ruby/pull/661
+
 Wed Aug  6 18:42:58 2014  Masaki Suketa <masaki.suketa@nifty.ne.jp>
 
 	* ext/win32ole/win32ole.c: separate src of WIN32OLERuntimeError
diff --git a/lib/set.rb b/lib/set.rb
index 0d9870ccd7..db57594d0a 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -100,11 +100,18 @@ class Set
   end
   private :do_with_enum
 
-  # Copy internal hash.
-  def initialize_copy(orig)
+  # Dup internal hash.
+  def initialize_dup(orig)
+    super
     @hash = orig.instance_variable_get(:@hash).dup
   end
 
+  # Clone internal hash.
+  def initialize_clone(orig)
+    super
+    @hash = orig.instance_variable_get(:@hash).clone
+  end
+
   def freeze    # :nodoc:
     @hash.freeze
     super
diff --git a/test/test_set.rb b/test/test_set.rb
index 559bc12897..169136f0d9 100644
--- a/test/test_set.rb
+++ b/test/test_set.rb
@@ -585,6 +585,28 @@ class TC_Set < Test::Unit::TestCase
     assert_equal 4, set.size
   end
 
+  def test_freeze_dup
+    set1 = Set[1,2,3]
+    set1.freeze
+    set2 = set1.dup
+
+    assert_not_predicate set2, :frozen?
+    assert_nothing_raised {
+      set2.add 4
+    }
+  end
+
+  def test_freeze_clone
+    set1 = Set[1,2,3]
+    set1.freeze
+    set2 = set1.clone
+
+    assert_predicate set2, :frozen?
+    assert_raise(RuntimeError) {
+      set2.add 5
+    }
+  end
+
   def test_inspect
     set1 = Set[1]