A planetary gear system (1) includes: a sun gear (5) having external teeth; a ring gear (9) having internal teeth; at least one planet gear (7) having external teeth meshed with the external teeth of the sun gear and the internal teeth of the ring gear. The planet gear (7) has, at its outer circumferential face, the external teeth which are formed of a pair of sets of teeth aligned in an axial direction and an annular groove (17) which is formed between the pair of sets of teeth. The planetary gear system (1) further includes: a fixed support shaft (23) provided for each of the at least one planet gear (7) and configured to rotatably support the planet gear (7) via a bearing, the bearing including a pair of rolling element rows (72) aligned in the axial direction; an oil supply path (LP) through which lubricating oil is supplied, at an inner side of the planet gear (7), to the pair of rolling element rows (72); and an oil discharge path (oil outlet 91) which extends through the planet gear (7) and through which the lubricating oil is discharged to the annular groove (17) from space between the pair of rolling element rows (72) which are provided at the inner side of the planet gear (7).