PURPOSE: To enable an almost complete secondary oxidation combustion to be performed without requiring a supply of energy of auxiliary fuel from outside except an initial firing or ignition by a method wherein sludges are granulated in advance and dried, partial carbonization, generation of combustible gas and gasification of tar substance and the like are performed rationally during primary combustion and then the final state of sludge is brought into contact with sufficiently heated secondary air.

CONSTITUTION: Sludge are granulated or dried in blocks, the particles are fed through a hopper 1 to make a complete filled condition and ignited through a secondary air flowing port 9. As the fire approaches a primary combustion zone 5, a closed state of an ignition heater 7 is released to feed primary air. At this time, the primary combustion and secondary combustion are regularly started. The primary combustion air causes each of gases to be flowed in such a direction as opposite to that of the movement of the ignited object and the secondary combustion air causes it to be flowed in an opposite direction, both discharged portions are placed at an intermediate part to cause combustible gas generated under a primary combustion to be ignited under a contact oxidation process at the surfaces of block- like or particulate ashes containing hot carbonate, and then carbon contained in the ashes is oxidized to make a more complete secondary combustion.