PURPOSE: To increase the oxygen concentration in a flame so that the combustion speed is increased and combustion efficiency is improved, by arranging in such a manner that oxygen-enriched air is used as primary air.

CONSTITUTION: Both oxygen-enriched air generating equipment 7 and secondary air feed fan are started to operate, while a screw feeder 10 of a pulverized coal feed equipment 9 is started to run. Oxygen enriched air, used as primary air, flowing from a nozzle 5a of an ejector 5, carries pulverized coal, passes through an inner cylinder 3, flows out of the end 3a and start burning. By this, combustion speed of fuel is increased and combustion efficiency improved, resulting in a smaller flow volume of primary air, as well as the reduction of sensible heat due to the primary air can be achieved.