The invention provides a planetary gear device for suppress increase of an axial length of the whole planetary gear device, and securing a large lubricant reservoir, and a maintenance method of a lubricant of the planetary gear device. A planetary gear device (G1) includes: a planetary gear (10); an internal gear (40) engaged with the planetary gear; a casing (50) integrated with the internal gear; a carrier (61) rotating relatively to the casing; a main bearing (71) arranged between the casing and the carrier; and a cover member (80) integrated with the casing and facing the carrier with a space (P1) in an axial direction. The carrier includes an outermost peripheral part (61A) on which the main bearing is arranged, and a recess part 65 (first recess part 66, second recess part 67) provided radially inward with respect to the outermost peripheral part and recessed on a side reverse to the cover member in the axial direction with respect to the outermost peripheral part