PURPOSE: To chuck a large-diameter workpiece liable to undergo elastic deformation in such a way as free from elastic deformation or chattering at the time of cutting the vicinity of the external surface of the workpiece.

CONSTITUTION: An outer chuck 10 having a chuck section 11 and a weight section 12 is pivotally fitted to the external edge of a main chuck 4 for holding the center axis 17 of a workpiece W via a support axis 9. When a chuck body is rotated, while the center axis 17 is being held with the main chuck 4, a centrifugal force acting on the outer chuck 10 causes the chuck section 11 to act toward a center, thereby holding the external surface of the workpiece W.