Cylinder mechanisms are provided for driving three jaw members each of which includes a cylinder hole having a long circular cross section, a piston which slides in the cylinder hole and has a long circular cross section, and a piston rod extending from the piston. The cylinder mechanisms are disposed at a position in the chuck body which is adjacent to corresponding jaw member in a state in which an axis of the cylinder hole in a direction parallel to a moving direction of the jaw member and a shorter diameter of the cylinder hole is directed in an axial direction of the chuck body.