PURPOSE: To enable an occurrence of white smoke to be prevented automatically by a method wherein a discharging and heating means in a wet type cooling tower is automatically controlled.

CONSTITUTION: A control device 1 measures temperature and humidity of surrounding air with sensors 15 and 16 and it is judged in reference to the measured data and a saturating temperature curve data inputted in advance whether or not white smoke is generated by a discharged gas A2 discharged from a wet type cooling tower 2. In the case that it is judged that there is a possibility that the occurrence of white smoke is found, a degree of opening of each of valves V1, V2 is increased to cause a penetration amount of hot water W1 at dry type heat exchanging parts 8a, 8b, thereby a temperature of the discharged gas A2 is increased to prevent occurrence of white smoke automatically. In the case that a heating calorie for heating the discharged gas A2 is poor only with the hot water W2, a switch 23 is closed and auxiliary heating means 10a, 10b acting as an electrical heater are operated and the discharged gas A2 is heated up to a predetermined temperature.