PURPOSE: To prevent electrochemical corrosion from occurring and to improve reliability by joining an aluminum-made external power transmission member by means of external fitting plastic joining over a serration section formed on the periphery of an iron-made internal power transmission member and by fitting a metallic ring member extending over the peripheral surfaces of both power transmission members.

CONSTITUTION: On the periphery of a shaft portion 10a of an iron-made yoke 10 as an internal power transmission member is formed a serration section 10b over which is fitted and plastically joined an aluminum-made pipe member 11 as an external power transmission member. Further, a metallic ring member 13 is press- or shrink-fit extending over the periphery of the pipe member 11 and over the periphery of the yoke 10 near the pipe member 11. As a result, electrochemical corrosion is prevented to improve reliability and joining force.