Abstract: A planetary gear device P includes a planetary carrier 3 integrally provided with a shaft portion 3.sub.1, a ring gear supporting member 7 having a ring gear 6 fixedly mounted on the outer periphery thereof, and an output member 10 spline-connected to the outer periphery of the shaft portion 3.sub.1. The right side of the ring gear supporting member 7 is supported on the left side of the planetary carrier 3 through a first thrust bearing 13, and the left side of the ring gear supporting member 7 is supported through a second thrust bearing 16 on the right side of a washer 15 secured to the shaft portion 3.sub.1, of the planetary carrier 3 by means of a circlip 14. With this arrangement, it is possible to reduce vibrations transmitted from the ring gear supporting member 7 of the planetary gear device P to the output member 10, to improve the mounting property of the planetary gear device P and to facilitate the management of dimensions of parts.