To form a center ring having a deep and wide groove without deforming a claw part by forging a conical semi-product having inner and outer tapered parts and a claw parts out of a ring stock, forming a groove in the height direction on one side and the other side of the semi-product through the cold-forging, and forming a circumferential groove in both tapered parts.

A ring stock formed through cold (hot) forging out of steel for mechanical structure is formed into a conical semi-product having an inner tapered part, an outer tapered part and a claw part through the cold forging. When the claw part of the semi-product is fitted to a groove in an upper edge of a knock-out ring 17, and a ram 26 is lowered, the semi-product is restricted by a lower punch 23, an upper die 27, an upper punch 32, and the knock-out ring 17, and a groove is formed in the inner tapered part of the semi-product by a projecting bar 25 in the longitudinal direction outside the lower punch 23. After the groove is similarly formed on the outer tapered part of the semi- product by a projecting bar of the upper die, a groove in the circumferential direction is machined in inner and outer tapered parts.