Speed reduction mechanism, and motor torque transmission device including the same

When an outer ring is fitted to an inner periphery of an input member, which defines a center hole, and an inner ring is fitted to an eccentric portion with clearances in a reduction-transmission mechanism, in a state where tooth tips of the input member contact bottomlands of a rotation force applying member on a line perpendicular to a second axis and a fourth axis, a size between the 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 a ball bearing and an inside diameter of the input member, which defines the center hole, a diameter difference between an inside diameter of the ball bearing and an outside diameter of the eccentric portion and an operating clearance of the ball bearing,