PURPOSE: To make it possible to drive a variable speed hydraulic rotary machine such as, for example, a pump with a fixed efficiency and an uniform performance irrespective of variations of water head, by driving the variable speed hydraulic rotary machine at a speed proportional to the square root of the mean value of water head which is determined by the condition of water level.

CONSTITUTION: A signal converter 6 delivers a rotational speed signal which varies in proportion to the square root of a water head signal indicating a water head H that is determined by the condition of water level for the water turbine of a pump. The rotational speed signal is delivered to a rotational speed controller 7 such as, for example, a cyclone converter which converts the signal into a signal having a frequency corresponding to a rotational speed with which the rotary machine is to be driven. The rotational speed of a generator motor 8 is controlled by the frequency, and the open degree of guide vanes in the pump turbine 10 is controlled by the above-mentioned rotational speed signal through a pump turbine governor 9.