Custom error page when accessing unauthorized page

Question

Iam using forms authentication along with role-based authorization in my web app. The problem Iam having is that when User1 who is already authenticated tries to access the page in the folder (HR) which is not authorized for, User1 is kicked back to login page instead of custom error page. Why does asp.net do this ? Is there any workaround to show custom error page to unauthorized users ? Thanks in advance. My web.config file is as follows.