Abstract: A stroke first air cylinder mechanism having a larger-diameter first piston and a larger-diameter first rod is incorporated in a casing, a chuck second air cylinder mechanism having a smaller-diameter second piston and a smaller-diameter second piston and a smaller-diameter second rod is coaxially incorporated into the first air cylinder mechanism, a base end portion of the second rod projects from the first piston and extends to such a position as to face a position detecting portion of the casing, a magnet is mounted to the base end portion of the second rod, and magnetometric sensor is mounted to the position detecting portion. By detecting an operating position of the second rod, opening and closing positions of jaw members can be detected.