PURPOSE: To provide a light ring gear supporting structure for epicyclic reduction gear, which can support a large rotating torque and which is flexible in the radial direction and in which the generation of friction is reduced.

CONSTITUTION: A sun gear, planet gears, and a ring gear are provided, and a ring-shaped fixing member 12, which is provided so as to surround the ring gear having a low radial rigidity and which is fixed to a fixation part 4, and plural long connecting members 14 for connecting the fixing member 12 and the ring gear are provided. The inner end 14a of each connecting member 14 is fitted to the ring gear with an equal space in the circumferential direction freely to be pivoted, and each connecting member is acutely extended in the tangent direction from the pivotal fitting point of the inner end, and the outer end 14b thereof is pivotally mounted on the fixing member freely.