A planetary gear reducer comprising: a sun gear having external teeth and fixed concentrically to a power input shaft; a ring gear having internal teeth and provided along an outer periphery of the sun gear; a plurality of planet gears having external teeth and meshing with the sun gear and the ring gear; a plurality of planet shafts, each of which is a rotational axis of a corresponding one of the plurality of planet gears; a planet carrier configured to support the plurality of planet shafts, and to output revolution of the plurality of planet gears around the input shaft as first power; and a cylindrical support being connected to the ring gear such that the cylindrical support is non-rotatable relative to the ring gear, the cylindrical support being configured to output rotation of the ring gear as second power, wherein the cylindrical support includes (i) a first cylindrical portion being formed at the ring gear side and (ii) a second cylindrical portion being formed at the output side, wherein the second cylindrical portion is a smaller-diameter cylindrical portion, and the first cylindrical portion is a larger-diameter cylindrical portion being formed such that a diameter thereof is larger than that of the smaller-diameter cylindrical portion, characterized in that the cylindrical support further includes (iii) a flexible structure provided at one part of the cylindrical support along a direction of a central axis of the input shaft between the first cylindrical portion and the second cylindrical portion, the flexible structure is curved radially inward and has a V-shaped longitudinal section formed by a front inclined portion (41 a) and a rear inclined portion, the front inclined portion is inclined radially inward so as to be tapered from an end of the first cylindrical portion, the rear inclined portion is inclined radially inward so as to be tapered from an end of the second cylindrical portion, wherein a wall thickness of the first cylindrical portion is smaller than a wall thickness of the flexible structure. a sun gear having external teeth and fixed concentrically to a power input shaft; a ring gear having internal teeth and provided along an outer periphery of the sun gear; a plurality of planet gears having external teeth and meshing with the sun gear and the ring gear; a plurality of planet shafts, each of which is a rotational axis of a corresponding one of the plurality of planet gears; a planet carrier configured to support the plurality of planet shafts, and to output revolution of the plurality of planet gears around the input shaft as first power; and a cylindrical support being connected to the ring gear such that the cylindrical support is non-rotatable relative to the ring gear, the cylindrical support being configured to output rotation of the ring gear as second power, wherein the cylindrical support includes (i) a first cylindrical portion being formed at the ring gear side and (ii) a second cylindrical portion being formed at the output side, wherein the second cylindrical portion is a smaller-diameter cylindrical portion, and the first cylindrical portion is a larger-diameter cylindrical portion being formed such that a diameter thereof is larger than that of the smaller-diameter cylindrical portion, characterized in that the cylindrical support further includes (iii) a flexible structure provided at one part of the cylindrical support along a direction of a central axis of the input shaft between the first cylindrical portion and the second cylindrical portion, the flexible structure is curved radially inward and has a V-shaped longitudinal section formed by a front inclined portion (41 a) and a rear inclined portion, the front inclined portion is inclined radially inward so as to be tapered from an end of the first cylindrical portion, the rear inclined portion is inclined radially inward so as to be tapered from an end of the second cylindrical portion, wherein a wall thickness of the first cylindrical portion is smaller than a wall thickness of the flexible structure.