PURPOSE: To allow a water level to come near a desired one by restraining the variation width of the water level to small one.

CONSTITUTION: Outflow from a water tank 4 and a water level in the same are measured respectively by a flow meter 5 and a water gauge 6. The number N of pumps outputted every time the outflow by a number-of-pumps alarm setter 7 reaches flow corresponding to the flow of the number of the respective pumps end three points of the upper limit value H, desired value M and lower limit value a water level alarm setter 8 are inputted to a number-of-pumps determining circuit 9 to determine the number of pumps and control the water level.