PURPOSE: To largely reduce NOX of a coal-fired boiler by supplying ultrafine powder having enhanced powder fineness as compared with fine powder coal to be supplied to a burner of other stage to an uppermost stage burner of burner units of multiple stages in a furnace.

CONSTITUTION: Fine powder coal mills 8a, 8b and 8c, 8d for supplying fine powder coals 5 finely pulverized from coal and having mean particle size of about 40-60μm are connected to lower stage burners 4a, 4b and intermediate stage burners 4c, 4d of a burner unit 4 provided in a furnace l of a coal-fired boiler. Ultrafine powder coal mills 18, 19 for finely pulverizing coal and supplying ultrafine powder coal 17 having a mean particle size of about 5-10μm are respectively connected to uppermost stage burners 4e, 4f. In this case, the mills 18, 19 have a rotary table 20 and a final pulverizing roller 21 for finely pulverizing coal under a large pressure, and a rotary classifier 22 for enhancing classifying performance. Thus, an ultrahigh temperature reducing atmosphere is formed in the furnace to reduce NOX generated at a lower side from the atmosphere.