diff --git a/ChangeLog b/ChangeLog
index ee026af1ec..eeecd69e5a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Mar 15 07:51:05 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* file.c (Init_File): do not define File::ALT_SEPARATOR on cygwin.
+
 Mon Mar 15 07:41:00 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* file.c (realpath_rec): use same cache.
diff --git a/file.c b/file.c
index 24b843e904..b6205cf899 100644
--- a/file.c
+++ b/file.c
@@ -5114,7 +5114,7 @@ Init_File(void)
     rb_define_singleton_method(rb_cFile, "split",  rb_file_s_split, 1);
     rb_define_singleton_method(rb_cFile, "join",   rb_file_s_join, -2);
 
-#ifdef FILE_ALT_SEPARATOR
+#ifdef DOSISH
     rb_define_const(rb_cFile, "ALT_SEPARATOR", rb_obj_freeze(rb_usascii_str_new2(file_alt_separator)));
 #else
     rb_define_const(rb_cFile, "ALT_SEPARATOR", Qnil);