PURPOSE: To reduce the number of times of starting/stopping a pump by increasing the speed of a motor until the pressure within a pressure tank is elevated up to a certain pressure that is higher than the prescribed feed water pressure after the lapse of a certain specified time from the time when a flow detector detects that the flow rate is less than the prescribed minimum flow rate.

CONSTITUTION: When a flow detector 7 attached to a feed water pipe 6 is operated due to the flow rate less than the prescribed minimum flow rate, the signal is sent to a control unit 12 which, if the signal stands on over a certain prescribed time, sends electric power to a variable speed motor 4 in order to increase the discharge rate of a feed water pump 5. Because of less water consumption in comparison with the quantity of the water conveyed through a feed water pipe 6, the rest of water flows into a pressure tank 9, which is branched from the pipe 6, elevating the water level in the tank 9. When a pressure detector 11, which carries operation even with prescribed feed water pressure, detects the prescribed upper limit pressure, it sends a signal to the unit 12 to stop the power supply to the motor 4. While the pump 5 stops its operation, the water to be used is supplied from the tank 9. And when the water level in the tank 9 is lowered, or the pressure within the tank is lowered, the motor 4 restarts its operation.