diff --git a/ChangeLog b/ChangeLog
index 09ff121300..77f199e598 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Feb 11 20:43:00 2010  Tanaka Akira  <akr@fsij.org>
+
+	* io.c (rb_io_oflags_modestr): return "r" for O_RDONLY|O_APPEND.
+	  [ruby-dev:40379]
+
 Thu Feb 11 19:19:21 2010  Kazuhiro NISHIYAMA  <zn@mbf.nifty.com>
 
 	* missing/alloca.c: s/RUBY_LIB/RUBY_LIB_PREFIX/ [ruby-dev:40395]
diff --git a/io.c b/io.c
index da3fc7bb7b..d92e104da7 100644
--- a/io.c
+++ b/io.c
@@ -4111,11 +4111,14 @@ rb_io_oflags_modestr(int oflags)
 #else
 # define MODE_BINARY(a,b) (a)
 #endif
+    int accmode = oflags & (O_RDONLY|O_WRONLY|O_RDWR);
     if (oflags & O_APPEND) {
-	if ((oflags & O_RDWR) == O_RDWR) {
+	if (accmode == O_WRONLY) {
+	    return MODE_BINARY("a", "ab");
+	}
+	if (accmode == O_RDWR) {
 	    return MODE_BINARY("a+", "ab+");
 	}
-	return MODE_BINARY("a", "ab");
     }
     switch (oflags & (O_RDONLY|O_WRONLY|O_RDWR)) {
       case O_RDONLY: