To heat a bag filter with reduced maintenance cost in the operation pause time of an incineration furnace by passing high temp. air heated by a waste gas in a waste gas treating equipment under operation through the bag filter of the waste gas treating equipment in the operation pause time in an incineration plant provided with 2 lines.

In the case that the 2nd line incineration furnace 1' pauses as the 1st line incineration furnace 1 is operated, an inlet damper 14' and an outlet damper 15' of the 2nd line incineration furnace 1' are closed and by-pass dampers 21' and 22' are opened to prevent the waste gas in a waste gas flow passage 2' from entering into the bag filter 8'. In this state, a damper 24 of the 1st line is closed and a dampers 27, 28 are opened to introduce the high temp. air after passing through a hot water heat exchanger 13 in the 1st line under operation into the bag filter 8' of the 2nd line through an air introducing pipe line 25. The air after passing through the bag filter 8' is returned to an air supply pipe 12 of the damper 24 post flow side through an air discharge pipe line 26. As a result, the energy which the waste gas possesses is effectively utilized.