While, it was not specified in the inventions disclosed in Japanese Patent Application Nos. HEI 3-357036 and HEI 3-350737 how to settle the point A.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)