Abstract: A pair of first conversion faces (55, 55) and a pair of second conversion faces (56, 56) are formed on the end face of a contact member (50) with which a movable member (now shown) comes into contact. The first conversion faces (55, 55) are separated by 180 degrees from each other in the circumferential direction of the contact member (50). The second conversion faces (56, 56) are separated by 180 degrees from each other in the circumferential direction of the contact member (50), and at the same time separated from the first conversion faces (55, 55) by a predetermined angle. The movable member is equipped with a pair of pressing projections which come into contact with the first conversion faces (55, 55) when the cover rotates to its closed position and come into contact with the second conversion faces (56, 56) when the cover rotates to its open position.