PURPOSE: To prevent a revolution speed command from deviating due to change in pump head by correcting a relation between revolution speed and rate of flow according to the pump head changing in accordance with the water level change in a water tank.

CONSTITUTION: A signal from water-level indicator 11 is supplied to a revolution speed adjusting function generator 10 by way of a pump head change arithmetic circuit 9, a component to correct a relation between running speed and rate of flow according to change in the pump head is obtained through the function generator 10, and the sum to an output of a function generator 6 is then obtained. A lag of running speed command can thus be kept from arising due to change in the pump head.