PURPOSE: To eliminate a change in pressure by alternately advancing piston rods in a pair of cylinders with difference in time to the pressure side of a plunger portion, and making the retreating speed higher than the advancing speed.

CONSTITUTION: Fluid pressure cylinders 21, 22 are connected to conduits 25, 26, and selectively connected to a hydraulic pump 29 and an oil reservoir tank 30 through directional control valves 27, 28. A piston rod 32 is advanced immediately before the advance stroke of a piston rod 31. The retreating speed of a piston head 23 is slower than the advancing speed and controlled by limit switches 48W52. Accordingly, high-pressure discharge pressure is operated immediately before the end of the advance stroke of plunger portions 33, 34 of the piston rods 31, 32, overlapping therewith, so that hydraulic pressure is kept from being lowered.