PURPOSE: To facilitate the simplification and miniaturization of structure of a chuck in making a pinch piece and a pusher so as to be mechanically driven without installing any air supply toute ranging from the side of a chuck holder to the side of a chuck body.

CONSTITUTION: Cam elements (19A, 19B) and (21A, 21B) are formed in a sliding body 11 installed in a chuck body 3 to be attached to a chuck holder 1, while a pair of pinch pieces 16A, 16B are driven for opening or closing by these cam elements (19A, 19B) and (21A, 21B) at the time of an axial movement of the sliding body 11, and a pusher 24 inserted into a through hole 23 of the sliding body 11 is constituted so as to be driven by spring force of an extruding spring 28 with the opening motion of these pinch pieces 16A and 16B.