To economically and efficiently treat a waste gas by introducing the waste gas into a gas cooler to make a low temp. saturated gas, heating it by a heating device and introducing it into a post stage dust collector to collect dust in the case of treating the waste gas discharged from a heating equipment.

The waste gas discharged from an incineration furnace 1 and passed through a heat recovery device 2 is sent to a wet dust collect 8 to collect the dust in the waste gas. A harmful gas such as hydrogen chloride or sulfur dioxide in the waste gas is also absorbed. The gas passed through the wet dust collector 8 is demoisturized from the saturated state in the gas cooler 10, in which cooling heat is radiated by a water cooler 12. The gas is again heated in the heating device 13 before flows-in to a poststage dust collector 4. A liquid phase sludge collected in the wet dust collector 8 is sent to a sludge treating equipment 15 through a line L5 and a fly ash 7 collected in a circulating water line of the gas cooler 10 and the poststage dust collector 4 is also sent to the sludge treating equipment 15 through a line L6 and L7.