A heavy duty lock case is mounted on a door and includes a latch coupled with a door frame for retaining the door in a closed position. An anti-theft latch can be operated to engage with the door frame. The latch cooperates with an anti-pick member to avoid the latch from being picked to disengage from the door frame when the door is in the closed position. When normally operated, an inner operating device or an outer operating device can be operated to disengage the latch from the door frame while disengaging the anti-theft latch from the door frame, permitting easy, convenient opening of the door.