A planetary gear starter with a molded synthetic resin ring gear supported by a frame is disclosed. The ring gear has longitudinally-extending ribs formed in its outer surface which confront longitudinally-extending inward projections formed in the inner surface of the frame. A cylindrical elastic member has an annular portion which is press fit over the ring gear and longitudinally-extending projections which fit into the cavities formed between adjacent ribs and inward projections in the frame. The projections of the elastic member serve as shock absorbers and elastically transmit torque from the ring gear to the frame. The elastic member has an annular portion which reinforces the open end of the ring gear.