Reduces backlash and clearance in a thrust direction of a worm when engaging a seat back to more securely prevent the generation of abnormal noise that before. A movable bearing 12 is pressed by cams 21, 22 that are pressing members, when the seat back is stopped at a predetermined angle, to approach a worm wheel 5. A ball 14 disposed in a recess 12a of the movable bearing 12 moves slightly along an inner surface of the recess 12a so a worm 4 is pressed to the worm wheel 5 side and pressed to the fixed bearing 11 side, and a latch member 23 that is a engaging member engages the worm 4 to inhibit rotation of the worm wheel 5 thereby securely holding that state. Since the movable bearing 12 moves to the worm wheel 5 side, the substantial movement range of the ball is wider compared to a structure that supports a ball only at a fixed bearing, and the range of displacement of the worm 4 by being pressed by the ball is substantially larger.