diff --git a/ChangeLog b/ChangeLog
index a3f18bc7a1..82f05a469c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu May 14 16:07:48 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/stringio/stringio.c (strio_ungetbyte): encoding should no
+	  be effective.
+
 Thu May 14 10:17:45 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* sample/test.rb (valid_syntax?): defaults to us-ascii.
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index bc605da2a7..9bb5ed8a2a 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -744,8 +744,37 @@ strio_ungetc(VALUE self, VALUE c)
 static VALUE
 strio_ungetbyte(VALUE self, VALUE c)
 {
-    NUM2INT(c);
-    return strio_ungetc(self, c);
+    struct StringIO *ptr = readable(StringIO(self));
+    char buf[1], *cp = buf;
+    long pos = ptr->pos, cl = 1;
+    VALUE str = ptr->string;
+
+    if (NIL_P(c)) return Qnil;
+    if (FIXNUM_P(c)) {
+	buf[0] = (char)FIX2INT(c);
+    }
+    else {
+	SafeStringValue(c);
+	cp = RSTRING_PTR(c);
+	cl = RSTRING_LEN(c);
+	if (cl == 0) return Qnil;
+    }
+    rb_str_modify(str);
+    if (cl > pos) {
+	char *s;
+	long rest = RSTRING_LEN(str) - pos;
+	rb_str_resize(str, rest + cl);
+	s = RSTRING_PTR(str);
+	memmove(s + cl, s + pos, rest);
+	pos = 0;
+    }
+    else {
+	pos -= cl;
+    }
+    memcpy(RSTRING_PTR(str) + pos, cp, cl);
+    ptr->pos = pos;
+    RB_GC_GUARD(c);
+    return Qnil;
 }
 
 /*
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index f456779880..69e3666cff 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -118,6 +118,22 @@ class TestIO < Test::Unit::TestCase
     r.close
   end
 
+  def test_ungetbyte
+    t = make_tempfile
+    t.open
+    t.ungetbyte(0x41)
+    assert_equal(0x41, t.getbyte)
+    t.rewind
+    t.ungetbyte("qux")
+    assert_equal("quxfoo\n", t.gets)
+    t.set_encoding("utf-8")
+    t.ungetbyte(0x89)
+    t.ungetbyte(0x8e)
+    t.ungetbyte("\xe7")
+    t.ungetbyte("\xe7\xb4\x85")
+    assert_equal("\u7d05\u7389bar\n", t.gets)
+  end
+
   def test_each_byte
     r, w = IO.pipe
     w << "abc def"
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 33bef0690b..8c72803b45 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -289,6 +289,21 @@ class TestStringIO < Test::Unit::TestCase
     f.close unless f.closed?
   end
 
+  def test_ungetbyte
+    s = "foo\nbar\n"
+    t = StringIO.new(s, "r")
+    t.ungetbyte(0x41)
+    assert_equal(0x41, t.getbyte)
+    t.ungetbyte("qux")
+    assert_equal("quxfoo\n", t.gets)
+    t.set_encoding("utf-8")
+    t.ungetbyte(0x89)
+    t.ungetbyte(0x8e)
+    t.ungetbyte("\xe7")
+    t.ungetbyte("\xe7\xb4\x85")
+    assert_equal("\u7d05\u7389bar\n", t.gets)
+  end
+
   def test_ungetc
     s = "1234"
     f = StringIO.new(s, "r")