PURPOSE: To restrict a generation of NOx having a high concentration when a non-combustible gas in a flame is to be burnt by a method wherein combustion air having a volume of a specified rate in respect to an amount of air for a full combustion is blown to a position near an extremity end of a flame.

CONSTITUTION: A combustion air 5c with a volume of 5W10% of an amount of full combustion air is blown toward a position near an extremity end of a flame 9, preferably toward an extremity end of a flame. In this way, a blowing direction blown from a top part of a combustion furnace is made in such a direction as being spaced apart from a central part of the flame 9, thereby a concentration of oxygen in a combustion area is decreased and thereby a generation of NOx having a high concentration is reduced. A part 6a of the combustion gas after passing through a heat exchanger 7 is recirculated and blown into a combustion furnace 2 at a location near a blowing port of combustion air 5c. With this arrangement, mixing of oxygen with non-combustible gas is promoted in a combustion furnace to enable a combustion with a low O2 to be performed, thereby a generation of NOx having a high concentration is more effectively reduced.