Abstract: A planetary speed changing device has an input shaft to which a torque is transmitted from an external device, an output shaft from which a torque of a reduced speed is output, eccentric members coupled to the input shaft through a flexible coupler which permits a relative radial displacement between the input shaft and the eccentric members, a plurality of externally toothed gears fitted on the eccentric members, an internally toothed gear having internal teeth formed by outer pins and meshing with the teeth of the externally toothed gears, inner pin receiving holes formed in the externally toothed gears, inner pins loosely received in the inner pin receiving holes, inner pin holder rings having holes tightly receiving the inner pins, and coupling means for coupling the inner pin holder rings to an output shaft.