PURPOSE: To prevent generation of white smoke to be generated by exhaust air from a wet-type cooling tower by making use of exhaust gas of a gas turbine.

CONSTITUTION: A smokestack 3 connected to an exhaust gas boiler 2 making use of exhaust gas of a gas turbine is connected to a cooling tower 5 by a duct 10, and a controlling damper 4 is provided in the position higher than the duct 10 in the smokestack 3, and a controlling damper 11 is provided in the duct 10. When the possibility of white smoke is generated after the atmospheric temperature is lowered, the controlling damper 4 in the smokestack 3 is closed, and the controlling damper 11 in the duct 10 is opened, thereby exhaust air 9 to be discharged from the upper part of the cooling tower 5 is heated by exhaust gas of the gas turbine 1, and white smoke is prevented from being generated.