Therefore, according to the present invention, based on the inventions disclosed in the above-cited Japanese Patent Application No. HEI 3-357036, a moving locus of teeth of the flexible external gear is obtained on a section of rotation of an end portion of the diaphragm side of tooth trace or on a section of rotation in the vicinity of outside of the end portion (wherein the section of rotation is referred to hereinafter as the limit section), and is smoothly connected to the envelope to form a composite curve, so that the composite curve is made to a basis of deriving tooth profiles of the gears.