diff --git a/ChangeLog b/ChangeLog
index ba433f64e3..411fa43e29 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Sep 28 10:06:38 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* stringio/stringio.c (strio_read): set ASCII-8BIT encoding
+	  when length argument is given.
+
 Mon Sep 28 01:28:17 2009  Yutaka Kanemoto  <kanemoto@ruby-lang.org>
 
 	* Makefile.in (miniruby): suppress duplication warning on AIX.
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 7514264f6a..a525dff235 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1168,7 +1168,7 @@ strio_read(int argc, VALUE *argv, VALUE self)
 {
     struct StringIO *ptr = readable(StringIO(self));
     VALUE str = Qnil;
-    long len, olen;
+    long len;
 
     switch (argc) {
       case 2:
@@ -1177,7 +1177,7 @@ strio_read(int argc, VALUE *argv, VALUE self)
 	rb_str_modify(str);
       case 1:
 	if (!NIL_P(argv[0])) {
-	    len = olen = NUM2LONG(argv[0]);
+	    len = NUM2LONG(argv[0]);
 	    if (len < 0) {
 		rb_raise(rb_eArgError, "negative length %ld given", len);
 	    }
@@ -1189,7 +1189,6 @@ strio_read(int argc, VALUE *argv, VALUE self)
 	}
 	/* fall through */
       case 0:
-	olen = -1;
 	len = RSTRING_LEN(ptr->string);
 	if (len <= ptr->pos) {
 	    if (NIL_P(str)) {
@@ -1209,6 +1208,7 @@ strio_read(int argc, VALUE *argv, VALUE self)
     }
     if (NIL_P(str)) {
 	str = strio_substr(ptr, ptr->pos, len);
+	if (argc > 0) rb_enc_associate(str, rb_ascii8bit_encoding());
     }
     else {
 	long rest = RSTRING_LEN(ptr->string) - ptr->pos;
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index df292e8a89..212315ae10 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -382,9 +382,12 @@ class TestStringIO < Test::Unit::TestCase
   end
 
   def test_read
-    f = StringIO.new("1234")
+    f = StringIO.new("\u3042\u3044")
     assert_raise(ArgumentError) { f.read(-1) }
     assert_raise(ArgumentError) { f.read(1, 2, 3) }
+    assert_equal("\u3042\u3044", f.read)
+    f.rewind
+    assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.read(f.size))
   end
 
   def test_size