PURPOSE: To make it possible to discharge a waste gas left unprocessed without contaminations by gasifying a fuel bed within a furnace by insufficient primary air and completely burning the same at the furnace top part.

CONSTITUTION: A solid fuel such as granular oil cokes or the like is supplied from the lower part of a combustion chamber 15 onto a grate 18 to form a non-fluidized thick fuel bed 31. Primary air adjusted to a quantity which is less than a stoichemical quantity by means of an air adjusting valve 21, and is blown into air chamber 17 to cause an incomplete combustion. Secondary air is ejected to top of the fuel bed 31 via an air adjusting valve 27, and air outlet port 29 and the above described incomplete combustion gas is completely burnt. This high-temperature gas is discharged out of a stack 41 via a smoke tube 33 and a smoke chamber 35. Since a secondary combustion part is cooled with boiler water from a water supply port 43, the combustion temperature is low and hence generation of NOX is reduced.