PURPOSE: To eliminate or reduce the no-water feed regions of two pumps of different characteristics when the pumps are operated in parallel with each other, by providing a rotational frequency controller.

CONSTITUTION: A flow rate signal generated by a flow rate detector 5 is entered into the controller 6, from which set rotational frequency signals are instructed to pump driving electric motors 3, 4 to run the pumps in parallel with each other. A signal or function generator 13 and an adder-subtracter 14 are provided. The flow rate signal is compared with a setpoint S by the controller 6 to generate a corrected rotational frequency signal, which is applied to the motor 4 for the pump 2 of smaller lift. The flow rate signal is applied to the function generator 13 to produce a signal corresponding to the flow rate signal. The produced signal is applied to the motor 3 for the pump 1 of larger lift through the adder-subtracter 14.