PURPOSE: To carry out the control of flow rate favorably without need for using a flow member by comparing values of the speed of rotation and water head with the border line of a pump operation control unable range, and moving a pump operating point.

CONSTITUTION: Values of the speed of rotation N of a pump detected by a rotation speed detector 25 and of a signal S1 from an actual water head calculator 23, are compared with the border line of a prestored range at which the control of pump operation is unable, to control a speed of rotation controlling device 27, in a pump 18 which controls the level of a suction water tank 15 to be constant. Thereby, the level of suction water can be favorably controlled to be constant by repeating the operation of the pump before and behind the pump operation control unable range, without need for providing a flow meter equipment.