The link member (10a) is connected to the lock lever (7) by a pin (23d). Thus, the link member (10a) is pulled to an upward direction by the upward movement of the lock lever (7). At this point, with the movement of the lock lever 7, the link members (10a) and (10b) operate toward a direction that increases the angle between the two, and the plunger (22a) connected to the linkage (15c) moves backward. Thus, when the plunger (22a) is unable to move with respect to the body of the electromagnetic solenoid (22) due to freezing etc., the linkage (15c) is unable to move. Therefore, in such a case, the lock lever (7) becomes inoperable.