PURPOSE: To reduce the quantity of NOX produced in combustion, while maintaining the thermal efficiency of a furnace, by a method wherein CO2 gas is added to enrich combustion air, thereby the temperature of a flame being stabilized and the local excessive rise in temperature within the furnace being prevented.

CONSTITUTION: The fuel and the combustion air are sucked, in required quantity, from a fuel gas pipe 2 and a combustion air pipe 3, respectively, into the fire furnace 1 of a boiler, where they are burnt. Moreover, a CO2 gas pipe 5 being connected to the middle part of the combustion air pipe 3, CO2 gas equal to 10W20% of the quantity of combustion air is supplied from the CO2 gas pipe 5 into the combustion air pipe 3, being added to enrich the combustion air, and is burnt in the fire furnace 1.