PURPOSE: To reduce the generation of injurious materials at the transient combustion time like an ignition rise time in the titled device provided with an inner and an outer flame tube and an outer tube which are arranged concentrically by a method wherein a carburetor part composed of heat insulating porous bodies is exposed to the primary combustion chamber between the inner and the outer flame tube.

CONSTITUTION: A vaporization part 1 sucks up fuel from a fuel tank 7 and is impregnated with it, the primary air is supplied from pores 2a, 3a and 6a provided respectively in the inner and the outer flame tube 2, 3 and an outer flame dish 6 and ignited and burnt with fuel in the vaporization part. At this time, combustion heat is used for vaporization of said fuel, the vaporiation fuel is carried to the secondary combustion chamber 11 through the primary combustion chamber 10 and burnt completely there by the secondary air supplied from inner and outer slits 8, 9. At this time, since the upper clearance B is made smaller than the lower clearance A in said primary combustion chamber 10, turbulences are given to the flow of combustion gas and the combustion air rising in said chamber 10 the mixing state and combustion performance are improved.