To attain water quality management that stably maintains the concentration of impurities in cooling water, in a cooling tower to a tolerance or lower by a simple and easy-to-use facility.
In maintaining the concentration of impurities to the tolerance or lower by controlling blown down amount by opening/closing an automatic water discharge valve 22 provided in a water discharge pipe 21 of the cooling tower 1, an inlet/outlet temperature difference of cooling water of a water cooler/heater 11, when the cooling water is circulated at a set flow rate at set load of the water cooler/heater, is represented as ΔT*. Reference opening time T2* for the automatic water discharge valve, necessary for discharging the blown down amount according to the evaporation amount of the cooling water, at this time in a control cycle T1, is set. Inlet/outlet temperature difference ΔT of the cooling water circulated and supplied to the water cooler/heater with respect to each control cycle is calculated. Opening time T2 of the automatic water discharge valve 22 is calculated by multiplying T2* by a ratio L of ΔT with respect to ΔT*, and according to the calculated opening time T2, the opening time of the automatic water discharge valve 22 is controlled in a next control cycle, so as to control the amount blown down.