A switch (50) is adapted to be On when a contact point (52) is at a position to be in contact with a cam surface (47) and Off when the contact point (52) is at a position corresponding to a non-cam surface (48). In the vicinity of the non-cam surface (48) of the cam surface (47), a rib (49) is provided which changes a direction of the contact point (52) when reciprocation of the cam member (43) is changed. Accordingly, even when the switch (50) comes into contact with the cam member (43) moving in a different direction, the direction of the contact point (52) is changed so as to be the same. Hence, the switch (50) is prevented from being affected by a hysteresis caused by difference in directions of the contact point (52).