PURPOSE: To carry out water feed control simply and at a low cost using a general industrial gauge by averaging weight for the water levels of plural water tanks to set a virtual tank level and controlling feed water based on said virtual water tank level.

CONSTITUTION: When plural water tanks are fed by means of one water feed system, the water level signals l1Wl3 of the plural water tanks 1W3 are inputted into an adder 4 to average weight and convert them into one virtual water tank level. Also, the output of the adder 4 is inputted into plural alarm setters 5W7 to set the upper and lower limit alarm for each or the water levels. And, the number of plural water feed pumps 9 are determined and operated in accordance with each of the alarm water level by means of a pump number controller 8 to control a water feeding quantity. thereby, the control of water feeding quantity can be carried out simply and at a low cost by using general industrial gauges.