Abstract: An intermediate molding (41) is formed so that a cylindrical arm-to-be portion (42) projects from the cylindrical outer surface of a body portion (13) of the arm-to-be portion (42). The arm-to-be portion (42) has a given thickness and a hole (42a) with a diameter greater than the inner diameter of a projecting connector portion (20) of an arm portion (15). A pair of flat face portions (42b) extending parallel to the axis of the body portion (13) are formed on the cylindrical outer surface of the arm-to-be portion (42). A tap (44) that has an external thread (43) formed along its cylindrical outer surface is inserted into the hole (42a) of the arm-to-be portion (42). Concave faces (49),(49) are concaves respectively formed on the opposing surfaces of an upper die (47) and a lower die (48) so as to have a shape like a half cylinder with a curvature corresponding to that of the cylindrical outer surface of the projecting connector portion (20).