PURPOSE: To ensure a steady supply of combusting air by dividing air space between combustor outer tube and combustor tube into primary air chamber and secondary air chamber and disposing primary air pipe and secondary air pipe respectively outside the combustor outer tube.

CONSTITUTION: A combustor outer tube 14 encloses a combustor tube with an air space inbetween. Fuel is heated by an electric heater 15 disposed under a grate 11 and starts burning by air in a refrigerator 17 which is supplied by a fan 16. The combusting air is branched off at a junction 18 into a primary air pipe 19 and a secondary air pipe 20, and enters into the combustor tube 10 from the grate 11 and secondary air hole 21. The air space between the combustor tube 10 and combustor outer tube 14 is divided into a primary air chamber 24 and a secondary air chamber 25 by a flange 22. Since the primary and secondary air are divided at a location which is not affected by the heat of the electric heater 15, the ratio between them is always constant, and so, even if the oxygen concentration inside the refrigerator 17 becomes low, the fuel can be maintained at a high temperature by activating the electric heater 15.