PURPOSE: To save the fuel for afterburning and effetively prevent white smoke by mixing the exhaust gas after wet denitration and desulfrization with air and passing through a mist separator and performing aferburning.

CONSTITUTION: The exhaust gas discharged from a boiler 1 is fed to an air mixer 5 through a denitration apparatus 2 and a desulfrization apparatus 3 and mixed with the air blown from an air fan 4 and cooled to the temperature in which humidity forms supersaturation state. Then, the exhaust gas is fed to a mist separator 6, and the condensed water vapour in the differential amount between saturation humidity and supersaturation humidity and mist are removed. Then, in a gas mixer 8, the exhaust gas is mixed with combustion high temperature gas which is generated in an afterburner 7, and the temperature is increased again to a fixed temperature. Through the procedures described above, the used amount of fuel for afterburning is saved, and the generation of white smoke can be prevented.