Abstract: Air is blown into a combustion furnace via an inlet disposed in the upper section of the furnace and the incoming air is made to flow in a fixed spirally descending path along the inner wall of the furnace proper. Material subjected to combustion is carried by the flow of air to the layer of burning material at the bottom of the furance by previously deposited combustible material, there to be burnt. By virtue of the action of the current of air, the combustion gas from the combustion is vigorously whirled upwardly in the same rotational direction as that of the air current in the annular region defined by the fixed path of the spirally descending air current. This ascending current of air is enclosed within the annular region defined by the descending current.