A reduction gear includes a shaft that has a gear portion and a ring gear that is mounted on the shaft at a position away from the gear portion. The shaft has at the position in the outer periphery thereof a plurality of stepped first teeth arranged along the circumferential direction of the shaft. Each first tooth has a lower portion and a higher portion that is higher than and contiguous to the lower portion in the axial direction of the shaft, and the higher portion having a step surface that is contiguous to the lower portion. The ring gear has in the inner periphery thereof a plurality of second teeth arranged along the inner circumference of the ring gear, and the second teeth are fitted to grooves between the first teeth through splines. The second teeth are set in contact with the step surfaces of the first teeth.