To enable sufficient incineration of even wastes sticking to a furnace bottom.

A furnace bottom 34 in a combustion chamber 11 is flat in substance. A lower air supply pipe 41 is provided from the furnace bottom 34. The air supply pipe 41 extends almost parallel with the furnace bottom 34 to be connected to a combustion fan. The air supply pipe 41 is provided with a link pipe 42. The link pipe 42 extends upward vertically from the air supply pipe 41 to the furnace bottom 34. An air nozzle having an air jetting port is mounted detachable freely on the link pipe 42.