A rim-shaped section of a teeth section with the same thickness among a bottomland and the same thickness as that of a web. An outward-radial-direction end of the web is connected to an inner end side of the rim-shaped section. In the rim-shaped section, a section from a teeth section connecting section of the web to an outer end of the ring-shaped rim-shaped section is connected to an outer circumferential surface of the boss by a plurality of first radial direction ribs. In the rim-shaped section, a section from the teeth section connecting section of the web to a tooth-width-direction center section is connected to an outward-radial-direction end of second radial direction ribs. The first and the second radial direction ribs are formed thinner than the web and the ring-shaped rim-shaped section.