PURPOSE: To enable to realize the constant flow amount discharge without pulsation or the like at all by a method wherein a motor for driving a plunger is driven, a three-way valve is operated after a predetermined period of time to begin the discharge and the driving motor is brought into a normal rotating condition within said predetermined period of time.

CONSTITUTION: Solenoid valves SOL2, SOL3, SOL4 are put ON with a delay of time T after the time of putting ON the driving motor M, a discharging nozzle 27 is opened, the three-way valve 24 is brought into a first position, a cylinder rear chamber 6 is comminicated with a liquid path 28 through the liquid path 12 and the liquid in the cylinder rear chamber 6 is discharged out of the discharging nozzle 27 by the left movement of the plunger 3 as shown by the arrow sign b. Thus, the three-way valve 24 is operated after the predetermined period of time from the starting of the driving motor M and the discharge is initiated, therefore, the driving motor M may be brought into the stationary rotating condition within said predetermined period of time, the affection of the variation of an discharging amount due to the irregular operation of the driving motor M upon starting will never be caused in the discharging fluid, and thereby enabling to obtain the constant flow amount discharge without pulsation or the like at all.