In these figures, the moving locus of Figure 6 is obtained at a section of rotation of a point 31 on the opening 3a of the teeth 30 of the cup-shaped flexible external gear 3. The moving locus of Figure 7 is obtained on a section of rotation at a center of tooth trace of the external gear, while that of Figure 8 is obtained at a section of rotation of a point 33 on an end portion of the diaphragm 3b of tooth trace of the external gear.