An internal meshing type planetary gear reducer made of plastic material constructed in such a manner that an external gear (24) fitted on an eccentric member (23) integrally formed on an input shaft (21) internally meshes with an internal gear (27), and inner pins (26) formed on the external gear integrally therewith are inserted in inner-pin holes (32) provided in a flange (31) formed at one end portion of an output shaft (22) so that rotation of the input shaft is reduced in speed and the speed-reduced rotation is transmitted to the output shaft. In the internal meshing type planetary gear reducer, each inner pin is provided with cutouts (26A) on inner and outer sides thereof in a radial direction of the external gear, or each inner-pin hole is provided with cutouts on inner and outer sides thereof in a radial direction of the flange, and only the inner pins which serve to transmit a torque at a predetermined range of the contacting location are arranged to be in contact with the inner-pin holes. With such construction, there is little resistance between the moving parts and the forcedly resistive sliding motion can be eliminated, whereby the torque transmission can be performed smoothly.
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