PURPOSE: To enable optional gripping and opening of an object to be gripped under rotation as desired by combining a chuck body attached to a hollow spindle and an annular rotating member which can be driven and rotated separately therefrom.

CONSTITUTION: An annular bushing 14 and an annular bearing metal 15 are fixed to the bottom end of a hollow spindle 13 through which an object 12 to be gripped is penetrated. An annular chuck body 18 formed radially with plural guide grooves 17 is connected via a bolt 19 to the metal 15. On the other hand, an annular rotating member 21 inscribed with a spiral screw 20 is interposed between the body 18 and the metal 15 freely rotatably with both. If a driving motor 24 is controlled in such a way that the angular speed of the member 21 changes relatively with the angular speed of the spindle 13, the member 21 rotates with respect to the metal 15 and the body 18 and gripping pawls 23 are opened and shut by meshing of the screws 20, 22 by which the body 12 under rotation is opened or gripped.