diff --git a/ChangeLog b/ChangeLog
index 16aeba6915..d943bb1b54 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Dec  8 17:52:24 2013  Kazuki Tsujimoto  <kazuki@callcc.net>
+
+	* object.c: [DOC] document Module#singleton_class?.
+
 Sun Dec  8 16:19:28 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* class.c (rb_get_kwargs): if optional is negative, unknown
diff --git a/object.c b/object.c
index 87e9608e76..3dc99367bb 100644
--- a/object.c
+++ b/object.c
@@ -2459,6 +2459,19 @@ rb_mod_cvar_defined(VALUE obj, VALUE iv)
     return rb_cvar_defined(obj, id);
 }
 
+/*
+ *  call-seq:
+ *     mod.singleton_class?    -> true or false
+ *
+ *  Returns <code>true</code> if <i>mod</i> is a singleton class or
+ *  <code>false</code> if it is an ordinary class or module.
+ *
+ *     class C
+ *     end
+ *     C.singleton_class?                  #=> false
+ *     C.singleton_class.singleton_class?  #=> true
+ */
+
 static VALUE
 rb_mod_singleton_p(VALUE klass)
 {