A first arm rotating within a predetermined angle about its rotational center is linked to a second arm rotating in a direction opposite to a rotation direction of said first arm. Two shrinking members are mounted on one end of the first arm. Two shrinking members shrink in opposite directions for each other each time when any one of them is energized. According to a switch of electricity to the two shrinking members, the first arm rotates to switch a rotational position thereof, with the second arm rotating to switch a rotational position of the second arm.