As mentioned before, the inventor of the present invention found that the above envelope is not but a similar curve obtained by reducing a moving locus at a ratio of 1/2, the moving locus being obtained when κ = 1 (non deviation) in the equation (2). However, it is insufficient to provide an effective tooth depth by using only this envelope. This is especially apparent when n = 1, that is, then the number of tooth difference between the rigid internal gear and the flexible external gear is 2.