Abstract: A ring gear and an outer race of a one-way clutch are linked and secured in the axial direction. The outer peripheral section of a disk-shaped supporting member engages teeth of the ring gear and supports the ring gear, and a tapered snap ring is mounted in an indented groove with a tapered surface formed in the ring gear so that the side surface of the outer peripheral section of the support member is in intimate contact with the end surface of the outer race, and the support member is thus secured and prevented from withdrawing. An oil sump for lubricant supplied from the inner radial side is formed on the inner peripheral part of the outer race defined by the intimately contacted outer race and the support member. The ring gear of the planetary gear unit rotates when the automatic transmission actuates, and the integral support member also rotates based on the rotation of the ring gear.