Custom error pages

I am having a problem with defining custom error pages in Apache.
I added the following line to my httpd.conf:

ErrorDocument 404 /main/error404.jsp

but nothing happens. The thing is, that if I give the full URL, like:

ErrorDocument 404 http://www.mypage.com/main/error404.jsp

then it works.
I thought that if a local URL, like the first one, is used - then it should be relative to the DocumentRoot (which is set in httpd.conf). Well, it is but it simply does not work!

What am I doing wrong???

Thanks in advance.
Gauti.
gauti@salt.is

I think the best way to do custom error pages is to define them in a .htaccess file. I did that and it worked fine.