PURPOSE: To miniaturize a suction water tank and moreover to suppress deterioration in efficiency at starting up and shutting down a pump and occurrence of oscillations and noises by controlling a discharge valve so as to keep a fluctuation of the water level coming in a constant width in accordance with a detection signal for the water level fluctuation.

CONSTITUTION: A fluctuation of the water level in a suction water tank 1 is outputted as a detected water level width signal by a water level signal generator 8 and is inputted in an arithmetic unit 9 to decide, as illustrated, whether or not the detection signal (in full line) comes in the water level width (in broken line) preset for steady operation of a pump. If it comes outside the preset level, a difference between the preset water level width and the detected water level width is calculated, and the result is sent to a function generator 10. A valve controlling device 12 therefore turns to control a discharge valve 6 so as to equalize an output from the function generator 10 and an output from a valve opening meter 11, thereby suppressing the water level fluctuation in the suction water tank 1 within a constant width.