PURPOSE: To expand the moving range of a pallet by using a spring for the clamping force of a clamp device while providing a quick coupling having a check valve which is connectable to said pallet, along a guide rail, and adjusting pressure to release and clamp said clamp device of a pallet, via said coupling.

CONSTITUTION: Plural clamp devices 14 which use springs for their clamping force, are provided on a pallet 2 which is moved along a guide rail 1 while holding a workpiece 7. Piping for an unclamping cylinder is introduced to a quick coupling 8 having a check valve. A quick coupling 9a having a check valve is provided connectably to the quick coupling 8 of the pallet 2, along the guide rail 1 and in a position for positioning the pallet 2. And, both of these quick couplings 8, 9a are connected to each other, and the workpiece 7 is mounted under an unclamped condition due to the hydraulic pressure of a hydraulic source and, then, is clamped due to the reduced pressure of the hydraulic source, to carry out machining. Then, the pallet 2 is moved along the rail 1 and stopped in the position of another quick coupling 9b and unclamped.