diff --git a/ChangeLog b/ChangeLog
index 969a49f97c..7b08560928 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Aug  1 14:59:04 2010  Tanaka Akira  <akr@fsij.org>
+
+	* ext/pathname/pathname.c (path_sub_ext): Pathname#sub_ext translated
+	  from pathname.rb.
+
 Sun Aug  1 10:23:48 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* lib/irb/init.rb (IRB.parse_opts): set VERBOSE to true when debug
diff --git a/ext/pathname/lib/pathname.rb b/ext/pathname/lib/pathname.rb
index b0639a4079..c30be4c948 100644
--- a/ext/pathname/lib/pathname.rb
+++ b/ext/pathname/lib/pathname.rb
@@ -39,15 +39,6 @@ class Pathname
     SEPARATOR_PAT = /#{Regexp.quote File::SEPARATOR}/
   end
 
-  # Return a pathname which the extension of the basename is substituted by
-  # <i>repl</i>.
-  #
-  # If self has no extension part, <i>repl</i> is appended.
-  def sub_ext(repl)
-    ext = File.extname(@path)
-    self.class.new(@path.chomp(ext) + repl)
-  end
-
   # chop_basename(path) -> [pre-basename, basename] or nil
   def chop_basename(path)
     base = File.basename(path)
diff --git a/ext/pathname/pathname.c b/ext/pathname/pathname.c
index 37fc87b500..ecfe11cfc8 100644
--- a/ext/pathname/pathname.c
+++ b/ext/pathname/pathname.c
@@ -167,6 +167,37 @@ path_sub(int argc, VALUE *argv, VALUE self)
     return rb_class_new_instance(1, &str, rb_obj_class(self));
 }
 
+/*
+ * Return a pathname which the extension of the basename is substituted by
+ * <i>repl</i>.
+ * 
+ * If self has no extension part, <i>repl</i> is appended.
+ */
+static VALUE
+path_sub_ext(VALUE self, VALUE repl)
+{
+    VALUE str = get_strpath(self);
+    VALUE str2;
+    long extlen;
+    const char *ext;
+    const char *p;
+
+    StringValue(repl);
+    p = RSTRING_PTR(str);
+    ext = ruby_find_extname(p, &extlen);
+    if (ext == NULL) {
+        ext = p + RSTRING_LEN(str);
+    }
+    else if (extlen <= 1) {
+        ext += extlen;
+    }
+    str2 = rb_str_dup(str);
+    rb_str_set_len(str2, ext-p);
+    rb_str_append(str2, repl);
+    OBJ_INFECT(str2, str);
+    return rb_class_new_instance(1, &str2, rb_obj_class(self));
+}
+
 /*
  * == Pathname
  *
@@ -364,4 +395,5 @@ Init_pathname()
     rb_define_method(rb_cPathname, "to_path", path_to_s, 0);
     rb_define_method(rb_cPathname, "inspect", path_inspect, 0);
     rb_define_method(rb_cPathname, "sub", path_sub, -1);
+    rb_define_method(rb_cPathname, "sub_ext", path_sub_ext, 1);
 }