In contrast to this, according to the present invention, the point A can be settled as follows. Where the deflection constant of the opening 3a of the flexible external gear is κa the value of κa < 1, and therefore in accordance with the expression (3),can be obtained. By applying this equation into the equation (2), the x and y coordinates of the point A can be settled. Further, for the purpose of later necessity, a value of ηa corresponding to the point A is obtained at this point from the equations (1) and (5).