when the temperature of the surface portion 1a reaches about 400 u00b0 c the palladium catalyst 3 carried on the combustion member 1 reaches a catalytic activation temperature then the palladium catalyst 3 operates such that catalytic combustion will occur in the catalytic combustion element 12 in this manner a switch over operation is performed from gas phase combustion to catalytic combustion since catalytic combustion does not generate flames an object to be heated can be disposed in the vicinity of the catalytic combustion element 12 to realize effective use of heat radiation this achieves a substantial reduction in fuel consumption as well as in the overall dimensions furthermore by increasing the flow rate of the air fuel mixture gas phase combustion can be started above the catalytic combustion element 12 while catalytic combustion still occurs in this case by causing catalytic combustion and gas phase combustion to occur concurrently high outputs can be obtained if the excess air factor is set to about 3 or the flow rate of air fuel mixture is further increased the flames generated in gas phase combustion are further increased the flames generated in gas phase combustion are further lengthened thus decreasing the temperature of the surface portion 1a of the combustion member 1 at this time catalytic combustion is far less likely to occur than gas phase combustion.