PURPOSE: To prevent temperature rise in the combustion zone where a flame is provided to keep combustion by supplying primary air for combustion from the end of the opening of either one of furnace walls and at the same time supplying the combustion exhaust gas from the intermediate section of the furnace wall and controlling the supplied amount of the combustion exhaust gas by means of the exhaust gas concentration in the furnace wall.

CONSTITUTION: The primary air for combustion is supplied from a chute hood 2 into a furnace wall 1, and the secondary air for combustion is supplied from first and second annular air supply chambers 8, 9 into the furnace wall 1. If, at this time, the temperature of a combustion zone C where a flame is provided for burning in high a damper 16 is automatically controlled by the signal of a secondary gas concentration detector 18, the damper responding to the signal, and the combustion exhaust gas is supplied to the combustion zone C where a flame is providied for burning, and gas concentration control, namely combustion control is easily done. Further, at the same time a damper 14 for air is also adjusted. Of course, if the dry zone A and the thermal dissolution, gassification zone and combustion zone D have a high gas concentration, gas concentration control, namely combustion control is carried out in the same way by the signal of first and third gas concentration detectors 17 and 19.