diff --git a/ChangeLog b/ChangeLog
index 87ecabc46f..070745e6cc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Dec 22 11:14:55 2004  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_mode_modenum): replace O_ACCMODE with O_RDWR.
+	  fixed: [ruby-dev:25273]
+
 Wed Dec 22 08:34:32 2004  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/dl/sym.c (rb_dlsym_initialize): extract internal pointers after
diff --git a/io.c b/io.c
index 8256b57782..e2547bc544 100644
--- a/io.c
+++ b/io.c
@@ -84,6 +84,10 @@ extern void Init_File _((void));
 
 #include "util.h"
 
+#ifndef O_ACCMODE
+#define O_ACCMODE (O_RDONLY | O_WRONLY | O_RDWR)
+#endif
+
 #if SIZEOF_OFF_T > SIZEOF_LONG && !defined(HAVE_LONG_LONG)
 # error off_t is bigger than long, but you have no long long...
 #endif
@@ -2469,7 +2473,7 @@ rb_io_mode_modenum(mode)
 #endif
             break;
         case '+':
-            flags |= O_RDWR;
+            flags = (flags & ~O_ACCMODE) | O_RDWR;
             break;
         default:
             goto error;