A planetary gear mechanism is desired in which pinion gears can be inserted between support portions on both sides in the axial direction of the pinion gears after the support portions on both sides in the axial direction of the pinion gears are coupled together. In this planetary gear mechanism, a coupling inner portion of a carrier coupling portion is located between a first sun gear and a second sun gear in the axial direction, and a carrier and the carrier coupling portion are formed so that each pinion gears can be inserted between a first shaft support portion and a second shaft support portion in the axial direction from the outside in the radial direction.