A planetary gear reduction system has a sun gear, a plurality of planet gears, a ring gear, and a planet carrier. The planet carrier has a first plate supporting one ends of planet shafts supporting the planet gears, a second plate supporting the other ends of the planet shafts, and a cylindrical drum connecting the first and second plates. The first plate has first portions supporting the one ends of the planet shafts; second portions connecting the first plate to the drum, the first and second portions being provided alternately in a circumferential direction; first cutouts each defined between the neighboring first and second portions, the first cutouts each extending radially inwardly from a circumferential edge thereof to a position adjacent a circle passing centers of the planet shafts; and second cutouts each defined radially inward of the second portion and between the neighboring two first portions, the second cutouts each extending radially inwardly from a first region outside the circle and a second region inside the circle.