To solve problems of conventional water supply secondary pump number control in which since the number control is performed by a system using a two-position regulator by load flow measurement or by use of a number control controller, attachment of a flowmeter to a pipe is needed, the cost is increased due to the necessity of the flowmeter body and incidental piping work and wiring work and, further, although appropriate flow control is performed by pump rotating speed according to the number control by adopting an inverter for water supply pressure control, pump shaft power is not taken into consideration since the increase/decrease number of pumps is determined based on a pump rated flow rate.

In the pump number control, setting of a flowmeter is dispensed with, and pressure control that is indispensable for variable flow water supply control is used. In response to a pressure signal from a terminal pressure transmitter or terminal differential pressure transmitter, a number control controller calculates the pump shaft power from a control output of a pressure regulator according to the frequency state of an inverter pump under operation, the number of operating pumps and the state of a control bypass valve, and increase/decrease stage determination is executed based on the calculation result to perform the pump number control with an appropriate pump shaft power. This number control includes a high energy saving effect.