For increasing load capacity the inventor of the present invention proposed a system using as the basis for the tooth face profile the curve obtained by similarity transforming the locus of motion at a reduction ratio of 1/2 over a prescribe range from the meshing limit point on the locus of motion based on the rack approximation of the tooth of the external gear relative to the internal gear determined by the shape of the wave generator (Japanese Patent Laid Open No. SHO 63-115923)