PURPOSE: To level the strength against an external force in the perpendicular direction to a disk surface by arranging a diameter direction member from a nipple hole in a disk periphery to an annular plate, and arranging a chord direction member positioned at the innermost circumferential side so as to pass through the vicinity of the annular plate.

CONSTITUTION: The number of projections 22 for stopping a high tension wire rod in an annular plate 23 is set to be half of the number of nipple holes in a periphery, and a disk comprises a diameter directional member R group, which is stretched between one nipple hole and the projections 22 on both ends among four projections facing the nipple hole, and plural chord direction members G1, G2 crossing the diameter direction member, and then the chord direction member positioned in the innermost circumferential side is arranged so as to pass nearby the annular plate 23. In this way, a lot of intersection points between the diameter direction members and the chord direction members exist near the center of the disk, and as the result the strength in a disk inner circumferential area is improved and the strength can be leveled all over the disk surface area.