On one side of a worm wheel (46) in an axial direction, first thinned portions (46e) recessed toward the other side of the worm wheel (46) in the axial direction are arranged. Further, on the one side of the worm wheel (46) in the axial direction, reinforcing portions (46h) reinforcing areas between the first thinned portions (46e) and tooth portions (45) of the worm wheel (46) are arranged. Therefore, it is possible to reduce a weight of the worm wheel (46) and to suppress occurrence of distortion of the tooth portions (45) by the first thinned portions (46e). In addition, it is possible to sufficiently increase the strength of the worm wheel (46) in the vicinity of the tooth portions (45) by the reinforcing portions (46h).