A transmission ratio variable mechanism is provided with: an input member and an output member capable of rotation about a first axis; an inner race for differentially rotatably coupling to each member; an outer race for rotatably supporting the inner race via a rolling element; and a transmission ratio variable mechanism-use electric motor for rotation-driving the outer race. A second axis which is a center line of the inner race and the outer race is inclined to the first axis. A first concave-convex engaging section for engaging one side end surface of the inner race and a power transmission surface of the input member corresponding to the one end surface are arranged. There is provided a second concave-convex engaging section for engaging the other end surface of the inner race and a power transmission surface of the output member.