To stably chuck even a thin-walled material having a split or crack by providing an opening/closing mechanism which radially opens/closes a chuck hand, and a centering means which is interlocked with the closing operation of the chuck hand and pushes the thin-walled material in the central direction.

A chuck hand 14 is lowered with its opened state in relation to a work W mounted on a set base and set in such a state that the top surface of the chuck hand 14 is positioned lower than the bottom face of the work W. Secondly, a rotary board is rotated so that a slide block 12 is slid to a fixed guide 11 and each chuck hand 14 is closed till its both ends are in contact with each other. Then, a pair of two pins 13 are brought in contact with the circumferential face of a work W so as to center the work W. This constitution can closely stuck the work sides to each other even the work has splits.