PURPOSE: To obtain large grasping force with a small amount of discharge oil, by directly coupling a pair of cramp vises, interlocked, with respective hydraulic cylinders and by allowing only one of the cylinders to operate till the point immediately before the cramping and both cylinders when cramping is completed.

CONSTITUTION: Cramping vises 1, 2 are directly coupled with hydraulic cylinders 3, 4, and their mutual coupling is made through an aligning link mechanism 10. When the cramping action is started, valves 26, 31 are set to the b side and the valve 27 to a side, and the valve 17 is changed over to the b side. At this time, pressure oil is supplied to oil chambers 3A, 3B, 4A, 4B, and only cylinder 3 is actuated according to the difference of the piston area to allow the vises 1, 2 to approach each other. When change-over to the valves 26, 31 is made at a position immediately before the cramping, the pressure oil is supplied only to the oil chambers 3A, 4A, and the cylinders 3, 4 are actuated to cramp the work to be processed.