An internally threaded carrying member is engaged with a screw thread formed on an operating shaft and is in frictional engagement with a moving member. A first projection is formed at one end part of the operating shaft and a second projection is formed on the internally threaded carrying member so as to correspond to the first projection. Upon the arrival of the internally threaded carrying member at one end of its stroke on the operating shaft together with the moving member as the operating shaft is rotated in one direction, the first projection and the second projection are engaged. Then, the internally threaded carrying member is rotated together with the operating shaft against a frictional resistance exerted thereon by the moving member as the operating shaft is rotated further in the same direction.
