PURPOSE: To easily perform fine adjustment of a delivery amount, by forming a piston integrally with a plunger, oil hydraulically driving the piston only at its return motion and directly applying pressure air for driving the piston at its feed motion.

CONSTITUTION: The rear end of a plunger 8 is secured to the front end part of a piston 34, and air pressure is applid to a rear end of the piston 34, while pressure oil, urged by air pressure, is acted on the front end part, and a set of flow passages, respectively having throttle valves 36 of variable type equipped with check valves 35 with a different stop direction, are formed between a tank 32, converting air pressure to oil hydraulic pressure, and a cylinder 33, housing the piston 34. Then the piston 34 is constituted such that its driving is performed by comoressed air for its feed motion and by oil hydraulic pressure, converted from the compressed air, for its return motion, thus the feed and return motions can be independently adjusted by the throttle valve 36. In this way, in accordance with kind and use of the metering delivery liquid, fine adjustment of delivery and suction speed can be easily performed.