Set Permission At Field Level

I want to set permission on Field Level.
Scenario : I have two fields Title And Name. If it is contributor then he can see both fields. And if he is Approver then he can see both fields but Name Field should be Readonly.

As i Need to achieve this with List Item Level Permission.

Reply me ASAP.
Thanks & Regards
Milan Chauhan

milanchauhan@live.com
http://sharepointnutcracker.blogspot.com