A ring that includes an inner peripheral surfaces that surrounds an outer peripheral surface of a pinion shaft provided through a pinion, a gear-side end surface, a bearing-side end surface, and an outer peripheral surface inclined from the gear-side end surface toward an outer peripheral edge of the bearing-side end surface is provided between the pinion and an inner race. When the length from an outer peripheral edge of the outer peripheral edge of the gear-side end surface to an axial line of the pinion shaft is L10, the length from a tooth root of a large gear to the axial line is L11, the length from an outer peripheral edge of a gear-side end surface of a large-diameter-side flange portion to the axial line is L2, and the length from the outer peripheral edge of the bearing-side end surface to the axial line is L3, L10