FIG. 13 illustrates a motion and a locus of one of teeth of a cup-shaped external gear having tooth profiles defined by the composite curve of FIG. 9 when moving with respect to a tooth groove of a rigid internal gear, showing the motion and locus at a central section of rotation of tooth trace of the cup-shaped flexible external gear.FIG. 14 illustrates a motion and a locus of one of teeth of a cup-shaped external gear having tooth profiles defined by the composite curve of FIG.9 when moving with respect to a tooth groove of a rigid internal gear, showing the motion and focus at a section of rotation of tooth trace on an end portion of the diaphragm of the external gear.FIG. 15 illustrates that a moving locus of teeth obtained through the rack approximation and a tooth profile at a tooth face can be approximated by circular arcs.FIG. 16 is an example of a standard rack of tooth profiles of the present invention.FIG. 17 illustrates a motion and a locus of one of teeth of a cup-shaped external gear having tooth profiles defined by the present invention when moving with respect to a tooth groove of a rigid internal gear, showing the motion and locus at a section of rotation on an opening of the cup-shaped flexible external gear (a section of non deviation).FIG. 18 illustrates a motion and a locus of one of teeth of a cup-shaped external gear having tooth profiles defined by the present invention when moving with respect to a tooth groove of a rigid internal gear, showing the motion and locus at a central section of rotation of tooth trace of the cup shaped flexible external gear.FIG. 19 illustrates a motion and a locus of one of teeth of a cup-shaped external gear having tooth profiles defined by the present invention when moving with respect to a tooth groove of a rigid internal gear, showing the motion and locus at a section of rotation of tooth trace on an end portion of the diaphragm of the external gear