PURPOSE: To eliminate wasteful operation of a pump by forcibly eliminating dusts and the like by utilizing water discharging force by the pump when the check valve of a pump side during stopping operation catch the dusts and the like so as to open the check valve even slightly, and rationalize water discharging operation when operation of the pump is restarted.

CONSTITUTION: Opening degree detecting sensors 7A, 7B are installed on check valves 4A, 4B facing to a plurality of pumps 3A, 3B. When opening condition of the check valve 4B on the pump 3B side on stopping operation is detected by the opening degree detecting sensor 7B during operation of either one pump 3A, the operation of the pump 3A during operation is stopped, and also operating condition is switched to operate the pump 3B corresponding to the check valve 4B in opening state.