PURPOSE: To ensure automatic control of a pump over its entire pump head without using a pressure tank by operating a diaphragm by means of a differential pressure generated in a resistance valve in order to operate a valve that closes a pressure receiving pipie for a pressure switch so that the switch is kept in an ON state.

CONSTITUTION: When the discharge rate is minute, the flow rate in a slit 62 of a resistance valve 61 is also minute. Thus the pressure at both sides of the valve 61 are nearly equal, and the pressure P1 and P2 at both sides of a diaphragm 71 are also nearly equal so that a pressure switch 9 is kept in an OFF state as shown in the figure. When a distant valve of a pipe 5 is opened, the pressure is lowered to set the switch 9 to ON, and a distant pump of a pipe 4 starts its operation, and the rate of flow passing the valve 61 increases and valve 61 moves upward and because of P1>P2, the diaphragm moves downward so that the opening of a pressure receiving 10 of the pressure switch 9 is closed by the valve 76 in order to maintain the ON state of the switch 9. When the distant valve of the pipe 5 is throttled to obtain P1P2, the valve 76 is restored, turning the switch 9 to OFF, and the pump stops.