diff --git a/ChangeLog b/ChangeLog
index 51a6690d13..0dba9bcc4e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Jul  4 11:52:12 2012  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* gc.c (ATOMIC_SIZE_*): 64bit Windows support.
+
 Wed Jul  4 11:11:28 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* eval.c (rb_frame_callee, rb_f_callee_name): fix to return the called
diff --git a/gc.c b/gc.c
index 82e250bdb6..72a2e2b575 100644
--- a/gc.c
+++ b/gc.c
@@ -61,11 +61,19 @@
 #endif
 
 #if defined _WIN32
-# define ATOMIC_SIZE_ADD(var, val) InterlockedExchangeAdd(&(var), (val))
-# define ATOMIC_SIZE_SUB(var, val) InterlockedExchangeAdd(&(var), -(val))
-# define ATOMIC_SIZE_INC(var) InterlockedIncrement(&(var))
-# define ATOMIC_SIZE_DEC(var) InterlockedDecrement(&(var))
-# define ATOMIC_SIZE_EXCHANGE(var, val) InterlockedExchange(&(var), (val))
+# ifdef _M_AMD64
+#  define ATOMIC_SIZE_ADD(var, val) InterlockedExchangeAdd64(&(var), (val))
+#  define ATOMIC_SIZE_SUB(var, val) InterlockedExchangeAdd64(&(var), -(val))
+#  define ATOMIC_SIZE_INC(var) InterlockedIncrement64(&(var))
+#  define ATOMIC_SIZE_DEC(var) InterlockedDecrement64(&(var))
+#  define ATOMIC_SIZE_EXCHANGE(var, val) InterlockedExchange64(&(var), (val))
+# else
+#  define ATOMIC_SIZE_ADD(var, val) InterlockedExchangeAdd(&(var), (val))
+#  define ATOMIC_SIZE_SUB(var, val) InterlockedExchangeAdd(&(var), -(val))
+#  define ATOMIC_SIZE_INC(var) InterlockedIncrement(&(var))
+#  define ATOMIC_SIZE_DEC(var) InterlockedDecrement(&(var))
+#  define ATOMIC_SIZE_EXCHANGE(var, val) InterlockedExchange(&(var), (val))
+# endif
 #elif defined HAVE_GCC_ATOMIC_BUILTINS
 # define ATOMIC_SIZE_ADD(var, val) __sync_fetch_and_add(&(var), (val))
 # define ATOMIC_SIZE_SUB(var, val) __sync_fetch_and_sub(&(var), (val))