PURPOSE: To lower unit price of water at the time of a small volume supply by starting and stopping a motor on the basis of measurement of frequency of start and stop of the motor within a time prescribed in advance and also of 2 points on the pump characteristics curve.

CONSTITUTION: When frequency of start and stop of a pump 4 within a time prescribed in advance, start of the pump 4 is conducted by a pressure switch 11 detecting an operating point X, and stop of the pump 4 is done by a detection step which detects an operating point Z having less volume of water on the pump characteristics than an operating point Y. When the pump 4 is stopped by a flow switch 6, the pump 4 is again started and stopped by the pressure switch 11, but when starting and stopping operations are conducted at a prescribed frequency, the pump 4 is started by the pressure switch 11 and it is stopped by the pressure switch 11 and the flow switch 6.