A fixed plate (15) is provided in a fixed metal frame (14). An opening and closing metal frame (17) having a movable plate (21) is attached to the fixed metal frame (14) so as to be openable and closable. The fixed metal frame (14) is provided with a pressurizing mechanism (23) applying a pressure in a closing direction generated by a spring (25) to the opening and closing metal frame (17) in a closed state. An actuating member (29) is movably provided at a position facing the pressurizing mechanism (23). Cam surfaces (30d, 32a) are provided between the actuating member (29) and the pressurizing mechanism (23). The spring (25) is deformed against an urging force thereof on the basis of operations of the cam surfaces (30d, 32a) in accordance with a movement of the actuating member (29), and the pressure applied to the opening and closing metal frame (17) is strengthened.
