PURPOSE: To eliminate pressure fluctuation generated at the changeover time of a hydraulic booster type continuous discharge extra-high pressure water pump.

CONSTITUTION: Spring back type single acting cylinders 1, 1' are placed parallelly in tandem, and a nozzle 8 is provided on the connecting pipe 7 of the respective cylinder tip parts. Solenoid valves (1), (2) are provided on the hydraulic circuits 9, 9' of both cylinders, and a throttle valve 13 is provided on the connecting pipe 14 of the hydraulic circuits 9, 9'. The individual hydraulic circuits 9, 9' and electric circuits for controlling the circuits 9, 9' are thus combined so that a small quantity of oil is fed to one cylinder 1' from the other cylinder 1 through the throttle valve 13 when the hydraulic piston 3' of one cylinder 1' reaches the upper limit.