To favorably lubricate a pinion gear bearing of a planetary gear device used in a vehicle automatic transmission or a vehicular automatic transmission without using a hydraulic pump, so as to increase the lifespan of the bearing and eliminate concern about the generation of creep between the bearing and the pinion gear.

A planetary gear device is constructed such that a deep groove ball bearing 33 is arranged around a pinion shaft 31 attached to a carrier 32, a pinion gear 29 is rotatably supported by the deep groove ball bearing 33, and a positional displacement prevention means is provided between the deep groove ball bearing 33 and the pinion gear 29.