Civicrm Custom Fields ACL Modification

I have created multiple custom data / fields for Civicase in Civicrm with drupal. I have one group of fields that is meant to be filled only by the administrator.. So that the simple registered members can see those fields and their values. For example. I submit a new Open Case for a car accident. Then the administrator log in and fill up a group of fields.. Cost.. Estimated time for repair of the car etc. I want to restrict the editing of those fields for me and only view them. I have set up the ACL Roles but the view function allow me to edit the fields.