PURPOSE: To provide a flue gas denitrification apparatus capable of efficiently making nitrogen oxides harmless without causing the corrosion of an apparatus due to ammonia.

CONSTITUTION: Some of the waste gas in a flue 4 is extracted from the upstream side of an ammonia injection nozzle 18 with an extracting pipe 16 and introduced to a vaporizer 17. The spraying pipe 19 which introduces an aqueous ammonia or its precursor 8 and the atomizing pipe 20 which introduces air or steam 9 is installed in the vaporizer 17, and the aqueous ammonia is sprayed into a mist. The temp. of the gaseous mixture 12 of the ammonia at the outlet of the vaporizer 17 is detected by a thermometer, and the temp. is controlled so as to keep ≥200°C to form an aqueous ammonia vapor. The gaseous mixture 12 of the ammonia from the vaporizer 17 is introduced into the upstream of the catalyst layer of the denitrification apparatus in the flue 4 and sprayed from a spraying nozzle 18. The steam of the aqueous ammonia is injected from the spraying nozzle 18 corresponding to the NOx amount in the flue gas and the NOx in the flue gas is reduced efficiently into a harmless nitrogen and water.