Provided is a planetary gear reducer including: a sun gear fixed concentrically to a power input shaft; a plurality of planet gears meshing with the sun gear; a ring gear meshing with the planet gears; a planet carrier configured to support a plurality of planet shafts, each of which is a rotational axis of a corresponding one of the planet gear, and to output revolution of the plurality of planet gears as first power; and a cylindrical support connected to the ring gear, non-rotatable relative to the ring gear, and configured to output rotation of the ring gear as second power. The cylindrical support includes, at one part along the direction of a central axis, a flexible structure which is curved radially inward and has a V-shaped longitudinal section. This configuration improves rigidity and prevents misalignment without increasing the weight of the gear reducer.