To prevent the other parts from becoming unattchable at a prescribed height, being inclined and loosened by burr by forming an annular projecting part having an outer peripheral surface as the extended peripheral surface of a projecting pin and an inner peripheral surface as the extended peripheral surface of a through-hole.

A fine gap 22 caused by wearing is formed between the inner peripheral surface 18A of the projecting part 18 at a die side and the outer peripheral surface 21A of the projecting part 21 of the projecting pin 15 by repeatedly forming a large quantity of hub cores. In the case of developing such the gap 22, the burr formed with the gap 22 between the inner peripheral surface 18A of the projecting part 18 at the die side and the outer peripheral surface 21A of the projecting part 21 of the projecting pin 15 is erected at the annular recessed bottom surface due to the projecting part 18 at the die side and the projecting part 21 of the projecting pin 15. The burr is only deformed in the recessed part while the front end part is pressed with a fitting foot part, and since the burr is not interposed between the fitting foot part and a boss plate as the forming part of the hub core, the fitting foot part abuts on the boss plate and is fixed.