PURPOSE: To effectively utilize heat by controlling thermal power while combustible wastes having different heat generating quantities are being completely burned.

CONSTITUTION: An air supply port 3, an exhaust port 4 and a fuel introducing port 2 are provided at the top of a furnace. Fuel is charged from the port 2, and deposited on a hearth 5. The deposited fuel 6 is ignited, the air if fed from the port 3, and oxygen is supplied to a flame 7. The flame 7 is completely burned, and the oxygen which is not consumed by the flame is arrived at the lower fuel 6 to continue burning. The burned gas is exhausted from the port 4. Since the larger the combustion increases, the more the oxygen quantity to be consumed by the flame 7 is increased, the oxygen quantity arriving at the fuel 6 is reduced to subside the combustion. On the contrary, the more the combustion decreases, the less the oxygen consumption of the flame 7 is decreased, the oxygen quantity arriving at the fuel 6 is increased, thereby accelerating the combustion.