To solve the problem caused by unevaporated water drops in a waste gas cooling column while suppressing the generation of dioxins and to prevent or suppress the occurance of hardly treatable fly ash.

The harmful components in the waste gas are removed by collecting the soot and dust in the waste gas in a first filtration type dust collector 51, then introducing the waste gas from which the soot and dust are removed into a hot water spraying and temperature lowering column 52 where hot water above the boiling point of water is sprayed, cooling the waste gas down to ≤200°C, then spraying chemicals to the waste gas and subjecting the waste gas to reactive dust collection in a second filtration type dust collector 4.