A plurality of convex portions that protrude radially outward from a plurality of positions separated in the circumferential direction are provided on the outer peripheral surface of a driven gear. Each convex portion has, in the circumferential direction of the driven gear, a rising surface that rises from a minimum diameter position to a maximum diameter position in the direction opposite the rotational direction of the driven gear, and a falling surface that falls from that maximum diameter position to a minimum diameter position that is adjacent to and in back of that maximum diameter position with respect to the rotational direction of the driven gear. The circumferential length of the falling surface is greater than the circumferential length of the rising surface.