The present invention is a chuck body (13) provided with: a hollow, substantially cylindrical fastener (11) having a tapered inner surface (13) in which the diameter of the hole gradually changes along the axial direction; and a chuck (21) having a divided end part (23) divided into two or three, the divided end part (23) having a tapered outer surface (24) that can enter or exit from the fastener (11) and that comes into contact with the tapered inner surface (13) of the fastener (11) when entering the fastener (11). When the divided end part (23) of the chuck (21) closes in the fastener (11) and holds the core (10), the angle of the tapered outer surface (24) of the chuck (21) relative to the center axis line of the chuck (21) is substantially identical to the angle of the tapered inner surface (13) of the fastener (11).