To provide a large locking force so as to save the electric power of a solenoid for releasing the lock by providing a link mechanism, which becomes the dead center condition when a lock bar is locked with a lock body and the closing condition of a shutter is locked.

In the case where a lock body 19 is locked in the maximum advance condition, when a shutter 4 is closed, a lock body 19 locks a lock bar 11 so as to lock the closing condition of the shutter 4. At this stage, a link mechanism 18 is in the dead center condition, and a line segment 30 for connecting each center of a first pin 23 and a third pin 27 and a line segment 31 for connecting each center of a second pin 25, a fourth pin 28 and the third pin 28 cross each other, and the line segment 31 forms a link, and generates a large resisting force in relation to the force in the direction crossing the line segment 30 so as to obtain a large locking force. When a solenoid is electrified, the line segment 30 is bent to the inverted V-shape in the link mechanism 18, and the resistance of the lock body 19 in relation to the force for releasing lock becomes remarkably small, and the shutter 4 can be easily opened.