To provide a structure which can suppress a surface pressure on the other edge part of an engagement part between a serration hole 12 and a male serration part 14 in order to reduce a shear stress applied to a part closer to the tip of the shaft 9 and, further, can easily secure the strength of a welded part between the base part 10a of the yoke 8a and the shaft 9.

An overhung cylinder part 16 having a thickness t₁₆ smaller than other parts of a base part 10a is disposed on the other edge part in the shaft direction of the base part 10a. Thereby, rigidity of the other edge part in the shaft direction of the base part 10a is lowered and a surface pressure on the other edge part in the shaft direction of an engagement part is suppressed in order to reduce a shear stress. Further, thickness difference t₁₆-t₉ between the overhung cylinder part 16 and a shaft 9 is reduced and, at the same time, the overhung cylinder part 16 and the shaft 9 are welded. Thereby, a penetration amount of the welded part between both portions 16, 9 is secured so as to be well-balanced and the strength of the welded part is secured.