diff --git a/ChangeLog b/ChangeLog
index ed10b57536..9ab5f85cf6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Aug 31 14:17:34 2008  Tanaka Akira  <akr@fsij.org>
+
+	* transcode.c (econv_primitive_convert): make two arguments,
+	  destination_byteoffset and destination_bytesize, optional.
+
 Sun Aug 31 14:12:06 2008  Tanaka Akira  <akr@fsij.org>
 
 	* transcode.c (make_econv_exception): error message simplified.
diff --git a/test/ruby/test_econv.rb b/test/ruby/test_econv.rb
index 52e8b97b4f..b1052dfc21 100644
--- a/test/ruby/test_econv.rb
+++ b/test/ruby/test_econv.rb
@@ -93,7 +93,16 @@ class TestEncodingConverter < Test::Unit::TestCase
     assert_equal("\xEF\xBD\xA1".force_encoding("UTF-8") * n, dst)
   end
 
-  def test_nil_destination_bytesize_with_nonnli_byteoffset
+  def test_nil_destination_bytesize2
+    ec = Encoding::Converter.new("Shift_JIS", "UTF-8")
+    n = 10000
+    src = "\xa1".force_encoding("Shift_JIS") * n
+    ret = ec.primitive_convert(src, dst="")
+    assert_equal(:finished, ret)
+    assert_equal("\xEF\xBD\xA1".force_encoding("UTF-8") * n, dst)
+  end
+
+  def test_nil_destination_bytesize_with_nonnil_byteoffset
     ec = Encoding::Converter.new("Shift_JIS", "UTF-8")
     n = 2000
     src = "\xa1".force_encoding("Shift_JIS") * n
diff --git a/transcode.c b/transcode.c
index 420f5752ff..d692b13d79 100644
--- a/transcode.c
+++ b/transcode.c
@@ -2321,6 +2321,8 @@ econv_result_to_symbol(rb_econv_result_t res)
 
 /*
  * call-seq:
+ *   primitive_convert(source_buffer, destination_buffer) -> symbol
+ *   primitive_convert(source_buffer, destination_buffer, destination_byteoffset) -> symbol
  *   primitive_convert(source_buffer, destination_buffer, destination_byteoffset, destination_bytesize) -> symbol
  *   primitive_convert(source_buffer, destination_buffer, destination_byteoffset, destination_bytesize, flags) -> symbol
  *
@@ -2345,9 +2347,12 @@ econv_result_to_symbol(rb_econv_result_t res)
  * destination_buffer should be a string.
  *
  * destination_byteoffset should be an integer or nil.
+ * nil means the end of destination_buffer.
+ * If it is omitted, nil is assumed.
  *
  * destination_bytesize and flags should be an integer or nil.
  * nil means that unlimited.
+ * If it is omitted, nil is assumed.
  *
  * primitive_convert convert the content of source_buffer from beginning
  * and store the result into destination_buffer.
@@ -2409,7 +2414,7 @@ econv_primitive_convert(int argc, VALUE *argv, VALUE self)
     unsigned long output_byteend;
     int flags;
 
-    rb_scan_args(argc, argv, "41", &input, &output, &output_byteoffset_v, &output_bytesize_v, &flags_v);
+    rb_scan_args(argc, argv, "23", &input, &output, &output_byteoffset_v, &output_bytesize_v, &flags_v);
 
     if (NIL_P(output_byteoffset_v))
         output_byteoffset = 0; /* dummy */