Planetary gear reduction system

A planetary gear reduction system (1) has a sun gear (5), a plurality of planet gears (7), a ring gear (9), and a planet carrier (11). The planet carrier (11) has a first plate (17) supporting one ends of planet shafts (13) supporting the planet gears, a second plate (12) supporting the other ends of the planet shafts, and a cylindrical drum (19) 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 (53) each extending radially inwardly from a circumferential edge thereof to a position adjacent a circle passing centers of the planet shafts (13); and second cutouts (55) 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 (Pc).