FIG. 9 illustrates the thus obtained composite curve. As shown in this figure, the composite curve C is formed of a portion of the envelope E and a portion of the moving locus L of teeth obtained at the limit section selected in the vicinity of the end portion as mentioned above. FIG. 10 shows five of moving loci of teeth at the respective sections of rotation of the respective positions in the direction of tooth trace of the flexible external gear, these moving loci being employed to obtain the envelope E