A planetary gear set including a sun gear; a carrier which rotatably supports multiple pinions meshed with the sun gear; and a ring gear in which internal teeth meshed with the pinions are formed in an inner peripheral surface and in which spline grooves are formed on an outer peripheral surface such that the ring gear serves as a hub of friction plates. A groove is formed along an entire circumference of part of the inner peripheral surface of the ring gear. Oil holes for introducing lubricating oil from the inner peripheral side to the outer peripheral side of the ring gear are formed such that one end of each of the oil holes opens at a bottom of the groove.