A speed reduction mechanism includes a bearing mechanism arranged on an axis of an output mechanism and including ball bearings that are arranged in parallel with each other in an axial direction of a differential case. The differential case is rotatably supported by the ball bearings of the bearing mechanism such that the entirety of an outer periphery of one of the ball bearings faces an inner periphery of a first housing element that accommodates the differential case and that is made of a material having a stiffness lower than a stiffness of a material of a rotation force applying member, and the entirety of an outer periphery of the other one of the ball bearings faces an inner periphery of the rotation force applying member.