FIGS. 6, 7 and 8 show a motion and

FIGS. 6, 7 and 8 show a motion and a moving locus l of one of teeth of the cup-shaped flexible external gear 3 with respect to a tooth groove of the rigid internal gear 2 of the flexing contact type gear drive 1, the motion and the moving locus being obtained by the rack approximation provided that the numbers of the both gears 2 and 3 become infinite. The motion and the moving locus of FIG. 6 are 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 (at a section of non deviation which have a normal deflection). The motion and the moving locus of FIG. 7 are obtained on a section of rotation at a center 32 of tooth trace of the external gear, while those of FIG. 8 are 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. The tooth profiles illustrated in these figures are a curved tooth profile consisting of convex and concave portions which is defined by the system disclosed in Japanese Patent Laid Open No.63-115943 as mentioned above. As is apparent from these figures, a continuous meshing is formed on the section of rotation at the opening 3a (at the section of non deviation) as shown in FIG. 6, whereas a tooth interference is occurred on the other sections of rotation of tooth trace as shown in FIGS. 7 and 8