To more reduce the amount of unreacted ammonia and further more reduce NOX concentration in flue gas without much labor technically and also with respect to an apparatus and further with the advantageous cost.

Flue gas 7 produced in a first step is first mixed in a second stage with a gaseous mixture medium containing no oxygen or containing a less amount of oxygen, mixed gas 9 produced thereby is forced to stay in a residence zone at least for 0.3 second and then mix it with secondary air 10 in a third step for complete combustion. Further, a reducing agent 11 is previously added to the secondary air 10.