PURPOSE: To prevent abnormal rise of pressure and thereby secure the safety by giving the pump such a characteristic as to work as a constant flow pump in a range from the start until steady operation is attained, and to work as a constant pressure pump when the pump pressure exceeds a certain specific value.

CONSTITUTION: The pushing force with a spring 17 is set at a value higher than the pressure which a piston 15 receives while the discharge pressure of the pump's pressure chamber 16 is in steady condition, and set so as to be dominating in a range from the start untill the steady condition is attained, and then the piston 15 is reciprocated in such a state as protruding the most from the plunger 11. When thereafter the pressure in the pump's pressure chamber 16 has risen to exceed a previously set value, the piston 15 under discharging action intrudes into the plunger 11 till a position where the discharge pressure is in balance with the pushing force of spring 17. Thus this pump works as a constant flow pump when the pressure is below a specific value and as a constant pressure pump when it has exceeded this value owing to action of the spring 17. Accordingly an abnormal pressure rise can be prevented.