PURPOSE: To provide a method by which an acceptable final emission can be produced by effectively reducing NOX, by forming a fuel-depleted fuel-air mixture by adding air to a gaseous effluent and a gas flow by passing the mixture over an oxidizing catalyst.

CONSTITUTION: Fuel and combustion air are supplied to a boiler room 10 through, for example, lines 14 and 16, respectively. When the fuel and combustion air are supplied to the room 10, the fuel and air are added to a combustion area 12 so that the fuel may become stoichiometrically excessive relatively to utilizable oxygen and the fuel is burnt. At this point of time, a flowing-out gas flow passes through a conduit pipe 18 still in an oxygen-depleted state. Then air is added to the gas flow in the pipe 18, and the resultant gas flow is led to a gas processing unit 26 and passed over an oxidizing catalyst. The gaseous effluent oxidized in the unit 26 is led to an economizer or a low-pressure waste heat boiler, a heat exchanger 28, etc., through a pipe 27 and the effective part of the heat content of the effluent is taken out. The boiler feedwater passes through the economizer 28, is heated through heat exchange with the gas, and led to a boiler 12 through a line 29. The cooled gas, on the other hand, is drawn out to a chimney 32 through a take-out pipe 30 and discharged to the atmosphere.