To solve the problems wherein a suction valve of a pump serving as a means for ejecting liquid easy to evaporate such as water in a negative pressure region of practical vacuum into the atmosphere or to the outside of a system does not open being stuck with a vacuum suction force and suction is not carried out in the first place, and secondly on the assumption that the suction valve is in an open state, when the liquid such as the water in the negative pressure region of practical vacuum is sucked into a cylinder, the occurrence of boiling and cavity following the pressure drop fluctuation occupies the content of the cylinder, and as a result, a rod piston idles all the time and the pump does not function.

An operating valve is provided in place of a conventional suction valve of a reciprocating pump, and a device is set to control so that the operating valve is opened in the appropriate position of suction and closed immediately before compression. In addition, the speed control of the suction retreat motion of the rod piston is performed corresponding to the natural inflow speed into the cylinder to attain normal suction, compression and discharge strokes.