PURPOSE: To reduce NOx, by a method wherein a combustion assist air supply port and a heating burner are provided in an inlet side exhaust gas dust and controlled by the oxygen concn. in a post-stream gas and the temp. in the duct.

CONSTITUTION: Fuel and air are thrown into a fire furnace 3 to be burnt. The combustion gas is subjected to endothermic cooling in the fire furnace and passed through a rear part flue 9 to be sent to a denitration apparatus 25. A bypass gas and combustion assist air are mixed in a mixer 16 and the mixed gas is burnt and heated by a heating burner 24 provided to the downstream side. The amount of combustion assist air is controlled by a control damper 32 so as to adjust the oxygen concn. in a coal economizer outlet flue 14 to a predetermined amount. The fuel of the heating burner 24 is controlled so as to hold the inlet gas temp. of the denitration apparatus 25 to a predetermined temp. NH3 is injected form a nozzle 20 in such a manner that the injection amount thereof is controlled so as to adjust NOx, NH3 at the outlet of the denitration apparatus to a predetermined amount.