To suppress relative rotation of a male screw member and a female screw member of a ball screw mechanism, with simple structure.

The ball screw mechanism 39 is constituted by engaging a lot of balls 111 in a spiral male screw groove 95a formed to an outer peripheral surface of the male screw member 95, and a spiral female screw groove 96a formed to an inner peripheral surface of the female screw member 96. A ball locking mechanism 114 for pushing the balls 111 with a lock pin 113 driven by a solenoid 112 and restraining the balls is provided, so that circulation of the balls 111 is suppressed and the male screw member 95 and the female screw member 96 can be locked to prevent their relative rotation. Since the ball locking mechanism 114 has simple structure constituted only by the solenoid 112 and the lock pin 113, the number of parts is reduced, layout property is improved, and cost is reduced.