PURPOSE: To reduce nitrogen oxide and carbon monooxide by a method wherein metal nets having their surfaces with major constituents of iron or iron oxide are installed in the midway of passage through which hot combustion gas is discharged out of the furnace.

CONSTITUTION: Primary air 3 is supplied through a dispersion plate 2 at the lower part of the main body of the boiler 1, coal and desulfurizing agent are supplied from the feeding pipe 4 to form a fluidized bed 5. In the fluidized bed 5 the coal is burnt and sulfur oxide produced along with the combustion of coal is reactioned with the desulfurizing agent such as lime etc. and solidified thereby and then discharged together with coarse ashes from the extraction pipe for the desulfurizing agent. At the free board above the fluidizing bed 5 is installed double staged combustion means for use in adding the secondary air 11, and at the free boards positioned above and below the feeding port for the secondary air 11 are installed the metal nets 10A and 10B. Thereby, the generated amounts of NOX and CO can be reduced.