PURPOSE: To improve operational moving ratio and machining accuracy by making it possible for a chuck, which is provided on a machine tool such as a lathe which holds a workpiece and rotate it by a motor, to hold both ends of the workpiece assuming the form of protruding beyond both sides of the chuck and to simultaneously machine both sides of the said workpiece.

CONSTITUTION: A both end opened cylindric collet sleeve 14, which is formed so that it can elastically expand and contract by supporting a rotating disc 10 on the inside circumference of a ring shaped part of a casing 1 through a bearing 9 and providing slittings 15 from its end surface, is arranged in the rotating center of the rotating disc 10. A clamp disc 19, which contracts a collet tube by impacting the tapered outside circumferential surfaces 16a, 16b of the collet sleeve 14 and makes it hold a workpiece 50 inserted into the collet tube, is concentrically screwed on the rotating disc 10 and it is equipped with a motor capable of rotating the rotating disc 10 forwardly and reversely and a stop plate for keeping the clamp disc 19 from rotating together with the rotating disc 10.