PURPOSE: To increase the efficiency of a boiler by a method wherein a total heat type heat exchanger for performing a heat exchange operation between air supplied to a combustion device and the waste gas is provided on the downstream side of a wet type desulfurizing device in an equipment for treating the waste gus from the combustion device through the desulfurizing device.

CONSTITUTION: The waste gas from the boiler 1 is introduced to an air preheating device 2 where it gives heat to combustion air supplied through a high pressure blower 9 and the total heat type heat exchanger 10 and is then introduced into an electric dust collector 3 to be deprived of dust. Further, the waste gas free of dust is introduced into the wet type desulfurizing device 4 where it is deprived of SOx contained therein and is humidified. After the above processes, the waste gas is induced by the operation of an induction type blower 5 and gives the water vapor (latent heat) and sensible heat thereof to the combustion air from the blower 9. Thus, by the above method it is possible to prevent the generation of white smoke from the waste gas and to improve the efficiency of the boiler.