Byte problems with excerpt tags?

Hi, I’m developing a WordPress theme for a portal website.
I’m using the following tag inside my index.php

<?php echo substr($excerpt,0,300); ?>

I need to define the word count to show latest posts on the index page.
The website uses both Japanese and English, the problem is that some letters shows up as strange characters.
Its supposed to look like this:

This is a post excerpt, click to read mo…

But instead it shows up as:

This is a post excerpt, click to read mo?…

Is it possible to remove the strange mark at the end of each excerpt?

Thanks for any help. ??