1. Atertiary negative-deviated flex

1. Atertiary negative-deviated flexing contact type gear (1) drive having a rigid internal gear (2), a cup-shaped flexible external gear (3) disposed inside the internal gear (2), and a wave generator (4) for flexing the external gear (3) into an elliptical shape so that a deflection occurred in the external gear (3) along a direction from a diaphragm (3b) to an opening (3a) thereof is proportional in value to a distance measured from the diaphragm (3b), a relative rotation of the both gears (2, 3) being generated by the rotation of the wave generator (4);