PURPOSE: To achieve light in weight, high in strength and a long lifetime.

CONSTITUTION: A cation electrodeposition film 11 is formed on the outer peripheral surface of an iron outer cylinder 1 press-fitted into a cylindrical holder 41 of a bracket made of a magnesium alloy material. A dichromate-processed film 43 is formed on the inner circumferential surface of the holder 41 in contact with the film 11. A rustproof film 5 excellent in sealing property is formed in such a manner as to cover a boundary between the holder 41 and an opening end of the outer cylinder 1. Both the cation electrodeposition film 11 and the dichromate-processed film 43 are excellent in electric insulating property. The rustproof film can prevent water from intruding into a contact surface, so as to prevent electrocorrosion from being generated between the inner circumferential surface of the holder and the outer peripheral surface of the outer cylinder, thus prolonging a lifetime. Additionally, the bracket made of a magnesium alloy material is light in weight and high in strength.