Figure 9 illustrates the thus formed composite curve L. In this figure, there are illustrated five moving loci in total including a moving locus ℓ a of teeth on a section corresponding to the state of non deviation and a moving locus ℓg of teeth on the section of rotation in the vicinity of outside of the end portion of tooth trace of the diaphragm (the abovementioned limit section), in addition to the respective moving loci ℓa, ℓb and ℓc shown in Figures 6, 7 and 8. Further, the respective points A, B, C and D of Figure 9 represent the places where the moving loci ℓa, ℓb and ℓc are contacted with the envelope e, respectively. Where the limit section is selected on the end portion of tooth trace of the diaphragm, the moving locus ℓg is in accordance with the moving locus ℓc.