PURPOSE: To lengthen the service life of catalyst by an arrangement in which the hot exhaust gas flows around the outer surface of a vaporizer, and by making the carrier coated on the outer wall surface of the vaporizer carry the oxidizing catalyst.

CONSTITUTION: An oxidizing catalytic layer 14 exists on the outer wall surface of a vaporizer 9, said catalytic layer comprising Pt catalyst carried by the carrier of sprayed alumina which is coated by γ-Al2O3 on the top of it. The catalytic layer 14 directly receives the heat conducted from the vaporizer 9, and shows an active state at a temperature which is maintained at about 300°C. The unburnt gas components in the exhaust gas are immediately reduced into CO2 and water vapor after contacting the catalytic layer 14, and so, the volume of the unburnt gas is reduced when the gas is expelled from exhaust pipe 6, alleviating the odor at the time of ignition. Meanwhile, since the pre-mixed gas is completely burnt on a flame hole plate 11 under steady burning conditions, there is no odor at all. As the catalytic layer 14 is not directly exposed to the flame, but rather is heated by the exhaust gas flow, abnormally heated spots are not formed. It is heated evenly, allowing a longer service life without a degradation of the catalytic performance.