Abstract: A wave gear drive comprises a rigid gear, a flexible gear engaging at a plurality of bulged regions thereof with the rigid gear, and a wave generator for causing the flexible gear to perform wave motion. The number of gear teeth between adjacent bulged regions differs between the two gears and the relative motion between the two gears is caused by shifting of the engagement regions. The teeth of the two gears are formed to satisfy the conditions that the shapes of imaginary rolling contact plate which performs the same motion as the aforementioned relative motion are determined to correspond to the loci described on planes fixed on the respective teeth by the respective momentary centers of relative tooth motion and that the common normal of the respective teeth at the meshing point passes through the momentary contact point of the rolling contact plates.