PURPOSE: To perform steady and no pulsation liquid conveyance at high volume effect by supplying a very small amount of working oil instantaneously linked with the reciprocation of a plunger and on the other hand, exhausting excess working oil.

CONSTITUTION: At the end of the suction of a main plunger 6, a very small amount of working oil corresponding to about 0.1W2% of the total displacement of a diaphragm 1 is injected instantaneously via an automatic supplying measn 5 from an oil reservoir 11 to a hydraulic working chamber 5. On the other hand, excess working oil and bubbles in the working chamber 5 are exhausted to the oil reservoir 11 by an automatic exhausting device 9 during the discharge stroke of the main plunger 6. By so doing, steady and no pulsation liquid conveyance can always be performed at high volume effect.