A wave gear drive comprises a circular rigid internal gear and a flexible external gear inside the rigid internal gear. A wave generator in the flexible external gear flexes the flexible external gear into an elliptical shape. The cross-section of the flexible external gear perpendicular to the axis at a point in the tooth trace direction of the flexible external gear is set as the main cross-section. The rigid internal gear and flexible external gear are gears with a module m, and the flexible external gear has a number of teeth that is smaller than the number of teeth of the rigid internal gear. The moving route M of a tooth of the flexible external gear with respect to a tooth of the rigid internal gear that accompanies rotation of the wave generator when meshing of the teeth in the main cross-section is approximated by rack meshing. On the entering side of the meshing of the teeth to the major axis of the moving route M, a first similar curve AC is obtained by similarity transformation of a pressure angle .alpha. orthogonal to the direction of the moving route M. The first similar curve AC is employed as the basic addendum tooth profile of the rigid internal gear. A second similar curve CB is obtained by rotating the first similar curve AC 180° about endpoint C, and by a similarity transformation in which the first similar curve AC is multiplied by a ratio of similitude (1−λ)/λ. The second similar curve CB is employed as the basic addendum tooth profile of the flexible external gear.