FIG. 11 shows how to introduce the tooth profiles of the present invention which is enable to realize a continuous contact from the composite curve C. Now, as a prescribed portion of the composite curve C, a curved portion C (A, B) is selected, ranging from its apex. A to a point B, wherein a depth or height of this curved portion C(A, B) is set to be two times the depth of the tooth face. The curved portion C(A, B) is subjected to similar transformation at a ratio of 1/2 with respect to the end point B, to thereby obtain a similar curve C1(M,B), which in turn is made to be a convex tooth profile at tooth faces of the rigid internal gear. According to an example shown in the figure, a curve C2(M, A) point-symmetrical with respect to a point M (this point being a pitch point or a datum point) to the curved portion C1(M, B) is made to be a convex tooth profile at tooth faces of the cup-shaped flexible external gear. Likewise, these curved portions C2(M, A) and C1(M., A) are employed to form each of concave tooth profiles of the respective deddendum portions of the rigid internal gear and the cup-shaped flexible external gear.