In a planetary gear device of an automatic transmission, including a sun gear, a plurality of pinion gears that engage with the sun gear, a carrier that rotatably supports the pinion gears, a ring gear having inner teeth formed on an inner periphery thereof for engagement with the pinion gears, a spline is formed on an outer periphery of the ring gear over a first portion that extends from one axial end of the ring gear to a given point between the one axial end and the other axial end, such that only the first portion of the spline engages with friction plates of a clutch. The ring gear further has a second portion that extends from the given point to the other axial end, which second portion has a larger thickness that the first portion of the ring gear that extends from the above-indicated one axial end to the given point.