PURPOSE: To clamp a workpiece very easily by positioning a stopper in accord with size of the workpiece in advance and, after the workpiece is mounted, rotating a screw shaft, thereby moving the stopper slightly.

CONSTITUTION: A workpiece is mounted on a support piece side in space between one side support piece and a stopper 15, and a screw shaft is moved along the shaft in the state that engagement between an engaging part 21 and a fixed part 23 of the screw shaft 19, and the stopper 15 is positioned in and around the workpiece. Next, a fixed part 22 is engaged with the engaging part 21 of the screw shaft 19, positioning the screw shaft 19 at the specified position. In this state, the screw shaft 19 is rotated and the stopper 15 is moved to the support piece side, whereby the workpiece is clamped and locked up by the support piece and the stopper 15. In this case, since the screw shaft 19 is rotated and the workpiece is clamped and locked after the stopper 15 is positioned in and around the workpiece in advance by movement of the screw shaft 19, a small rotation frequency of the screw shaft 19 will suffice.