Figure 10 shows how to derive the tooth profiles of the present invention from the composite curve L. Now, a prescribed portion of this curve is selected from a point Ato a point E. The starting point A is a point where the moving locus ℓa of teeth on the section of rotation of the opening 3a of the flexible external gear 3 is contacted with the envelopee. The terminating point E is a point where the value in y-ordinate is equal to the two times of working tooth depth, this pint being usually located on the moving locus ℓg of teeth on the limit section as mentioned above.