Abstract: A chuck for a tool is provided, which firmly fastens the working tool and can maintain the fastened state of the tool even when added vibration to be caused during operation. A plurality of jaws are obliquely provided relative to the axis of a chuck body, a grip sleeve is provided close to the rear end portion of the chuck body in unmovably fitted condition relative to the chuck body, a male screw portion is formed on the outer peripheral surface of each jaw, a nut sleeve is rotatably mounted to the chuck body, a female screw portion is provided on the inner peripheral surface of the nut sleeve, a rotational sleeve is provided in axially slidable condition to the chuck body but unrotatably engaged to the said nut sleeve, and the male screw portion and the female screw portion are engaged with each other, and whereby when said rotational sleeve is rotated by hand, the jaws are advanced/retracted to thereby clamp/release the tool.