An actuator containing a drive mechanism that moves a locking pin forward and backward. Said drive mechanism is provided with the following: a holder that holds the locking pin; a cam member that has a cam gear; and a power-transmitting member in which a lower gear to which power from a motor is transmitted and an upper gear coupled to the cam gear are formed. The cam is offset from the center of rotation of the cam gear. The holder is pushed in a forward/backward direction by reciprocal motion of the cam within a range that constitutes less than a full revolution about the center of rotation of the cam gear. A region into which part of the lower gear fits and a region through which the cam moves are provided in a space above one surface of the cam gear.