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 .