PURPOSE: To enable to use gas fuel combined with liquid fuel and vice versa, by providing a fuel conducting pipe to supply gas fuel and a mechanism to evaporate fluid fuel and supplies it together with the primary air, on the upper stream side of combustion catalyzer.

CONSTITUTION: The porous asbestos 2 is closely contacted with the rear side of a combustion catalyzer 1, in which a catalyzer heater 3 to heat the catalyzer 1 is provided, and at the same time, the wire nets 4 and 5 in the front and in the rear parts, for the purpose to prevent the catalyzer 1 and the asbestos 2 from coming off, are assembled in an integral body, so as to pinch catalyzer 1 and the asbestos 2 between them. The wire-nets are fixed by franges 7 and 8. A plurality of port 9 are provided to the rear space of the rear wire net 5 to supply gas fuel to the catalyzer 1 uniformly, and also a gas fuel supply pipe 10, interconnected to the gas fuel pipe 12 on the way of which a pressure adjuster 11 is built-in, is provided. When gas fuel is used, the fuel decreased its pressure down to the specified value by the pressure adjuster 11 is injected from the ports 9. On the other hand, when the liquid fuel is used, the fuel is vaporized into a vaporizer 16, having a gas heater 15 within, by an electro-magnetic pump 13 through liquid fuel feed pipe 14.