PURPOSE: To reduce the amount of release of harmful gas by supplying combustion air of high oxygen concentration at a specified stage according to actual demands of a combustion process.

CONSTITUTION: Dust (refuse) 3 is transferred to the downstream side of a grate 1 in a combustion process from a bringing-in shoot shaft 2 and slag 4 drops in a slag removing unit. In order to achieve as complete incineration of the dust 3 being present in a combustion area on forced draft regions 7a-7c as possible with a small quantity of charged combustion air, oxygen-enriched air is supplied and the dust is burned under the conditions of high temperature and longer gas convection time. Since the amount of oxygen demanded for combustion becomes low considerably in regard to the residues in forced draft regions 7d and 7e being a combustion completion area, inactive gas is added to the combustion air for the last forced draft regions so as to reduce the oxygen concentration. Moreover, a first secondary combustion air supply pipe 9 and a second secondary air supply pipe 11 are disposed above the grate 1 so as to conduct secondary combustion of waste gas, and the waste gas is combusted completely by regulating the amount of supply of oxygen.