Abstract: A cup-type harmonic gear drive comprises a rigid circular spline, a cup-shaped flexspline disposed inside the circular spline and a wave generator means fitted inside the flexspline for flexing the flexspline into elliptic to engage it with the circular spline and for rotating the engagement portions to thereby produce relative rotation between the splines. The flexspline has a tube, a disc-shaped diaphragm connected integrally to the end of said tube to define the cup bottom, and a thick boss formed on the center of the diaphragm. The diaphragm has a root portion from the boss, a thickness of which is set at least about three times the minimum thickness of the diaphragm. The outer profile of the diaphragm is defined, when viewed along a plane including the axis of the flexspline, by a plurality of curves having a different curvature. These curves are arranged from one having a larger curvature from the root protion to the tube portion of the diaphragm.