Abstract: A lock device is provided in which the locking bolt can be actuated by turning the outside lock cylinder or the thumbturn to releasably interlock the housing fixed to the inside surface of the door with a strike attached to the door frame. An inside lock cylinder is rotatably provided in the thumbturn and is designed to releasably lock the thumbturn. The lock device can not be dismounted from the inside of a room by an unauthorized person since all screws for mounting the housing are concealed within the housing, when the door is locked by the outside lock cylinder and the thumbturn is also locked with the inside lock cylinder.