PURPOSE: To make it possible to prevent the generation of pulsating flow upon the supply and discharge by high pressure liquid and reduced pressure liquid, by communicating both cylinder chambers of a cylinder of a bi-rod type with a pressure medium supply and discharge section through a selector valve and a flow control mechanism, and by controlling the flow of the pressure medium with the use of the flow control mechanism.

CONSTITUTION: During the liquid discharge stroke of a pressure reducing cylinder 17, the liquid supply stroke of a pressure reducing cyliner 20 is terminated since a selector valve 27 is switched to its neutral position and a high pressure supply valve 21 is closed, and then, the pressure reducing stroke of the pressure reducing cylinder 20 is initiated when the selector valve 27 is switched to its second position. Thereafter, simultaneously with the switching of the selector valve 27 to the neutral position, a pressure reducing valve 19 is closed to terminate the liquid discharge stroke of the pressure reducing cylinder. Accordingly, no pulsating flow is brought about even if two pairs of pressure reducing cylinders are operated, simultaneously.