diff --git a/ChangeLog b/ChangeLog
index 6da4788141..2d411873e8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Nov 21 15:58:43 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* io.c (read_all): fix: false negative invalid byte seequence
+	  on reading from pipes. [ruby-dev:39743]
+	  fix: assigin the variable 'pos' as relative value from recent pos.
+
 Sat Nov 21 14:44:16 2009  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* file.c (file_path_convert): delay getting UTF8-MAC encoding
diff --git a/io.c b/io.c
index 7b0afa44cd..7acd28571b 100644
--- a/io.c
+++ b/io.c
@@ -1742,7 +1742,7 @@ read_all(rb_io_t *fptr, long siz, VALUE str)
 	}
 	bytes += n;
 	if (cr != ENC_CODERANGE_BROKEN)
-	    pos = rb_str_coderange_scan_restartable(RSTRING_PTR(str) + pos, RSTRING_PTR(str) + bytes, enc, &cr);
+	    pos += rb_str_coderange_scan_restartable(RSTRING_PTR(str) + pos, RSTRING_PTR(str) + bytes, enc, &cr);
 	if (bytes < siz) break;
 	siz += BUFSIZ;
 	rb_str_resize(str, siz);
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 976aae8c2e..4d1a62381b 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -558,6 +558,12 @@ EOT
       assert_equal(eucjp, r.read)
     }
 
+    with_pipe("UTF-8") {|r,w|
+      w << "a" * 1023 + "\u3042" + "a" * 1022
+      w.close
+      assert_equal(true, r.read.valid_encoding?)
+    }
+
     with_pipe("UTF-8:EUC-JP") {|r,w|
       assert_equal(Encoding::UTF_8, r.external_encoding)
       assert_equal(Encoding::EUC_JP, r.internal_encoding)