PURPOSE: To contrive to strikingly reduce the running cost of a cooling tower by eliminating the supply of supplementary water beyond necessity and the unnecessary feeding of cooling water treatment chemicals by a method wherein the optimum amount of supplementary water is calculated by detecting the temperatures of cooling water and of the outside air.

CONSTITUTION: The titled supplying device consists of a temperature detector 5 to detect the temperature of cooling water at the inlet of a cooling tower, a temperature detector 6 to detect the temperature of the cooling water at the outlet of the cooling tower, a temperature detector 7 to detect the dry bulb temperature of the outside air, a temperature detector 8 to detect the wet bulb temperature of the outside air, an operator 9 to set the optimum amount of supplementary water to the cooling tower based upon the temperatures, which are measured by the temperature detectors 5W8 and inputted in the operator 9, and a flow rate detector 10 and a flow control valve 11, both of which are arranged in a supplementary water feed line 4a, and the like. The flow rate detector 10 and the flow control valve 11, both of which are connected to the operator 9, continuously monitor and control the flow rate to the cooling tower based upon the optimum amount of supplementary water to the cooling tower set by the operator 9.