PURPOSE: To aim at preventing the accuracy of working from lowering and as well at preventing a risk due to coming off of a workpiece, by connecting an accumulator to a hydraulic cylinder, and as well by visually or automatically detecting the critical pressure hold compensation of the accumulator so that the working is stopped.

CONSTITUTION: A workpiece is attached to a jig by supplying a hydraulic pressure of 100kg/cm2 into a hydraulic cylinder Hc. When the working of the workpiece is carried out in the position of a pallet P1 after the coupling between high pressure joints J1, J2 is released, oil slightly leaks so that the hydraulic pressure tends to be lowered. However, oil with an amount corresponding to the amount of the leak oil is fed from the storage chamber 4 of an accumulator Ac to compensate the lowered hydraulic pressure. If the oil leaks is lasted, the oil in the storage chamber 4 is lost out, and the critical pressure hold compensation is established. However, this critical pressure hold compensation may be visually observed by the retracted degree of a measure bolt, and as well an alarm is issued in accordance with a detection signal. Further, the working is automatically stopped, thereby the safety of the working may be enhanced.