However, it is insufficient to provide an effective tooth depth by using only this envelope. Therefore, according to the present invention, phantom teeth are provided in thevicinity of the end portion 33 of tooth trace of the diaphragm, and a moving locus is obtained at a section of rotation of the phantom teeth (this section is hereinafter referred to as a limit section) through the rack approximation as like as in the above. The obtained moving locus is connected to the above envelope to thereby form a composite curve, based on which the tooth profiles of the present invention are created. When the phantom teeth are provided in the vicinity of the outside of the end portion of tooth trace of the diaphragm, a desired tooth depth can be obtained, maintaining a continuous contact all over the tooth trace along the direction of tooth trace. On the other hand, the phantom teeth are provided on the end portion 33 of tooth trace of the diaphragm, a continuous meshing of the tooth profiles within the section of rotation on this position can be utilized. The latter fact is proposed at the first time here in this patent application.