When a bearing includes outer and inner rings and the outer ring is fitted to the inner periphery of an input member, which defines a center hole, with a clearance and the inner ring is fitted to an eccentric portion with a clearance, a size between a second axis and a third axis is set to a size that is smaller than or equal to half of a size obtained by adding a diameter difference between an outside diameter of the bearing and an inside diameter of the input member, which defines the center hole, a diameter difference between an inside diameter of the bearing and an outside diameter of the eccentric portion and an operating clearance of the bearing in a state where the input member has been moved to contact the housing on a line perpendicular to the second axis and a fourth axis.