PURPOSE: To elevate the temperature of the evaporating section of fuel while keeping the durability of materials constituting a combustion chamber and the thermal efficiency of an apparatus, to reduce odor on fire extinguishing and to prevent the deposition of tar by mounting a water drum section in the side wall of the combustion chamber at a position higher than the lower surface of the combustion chamber and upward of a position lower than the upper end of a combustion assisting cylinder.

CONSTITUTION: Furl is fed from a pump 7, and forwarded into a combustion assisting cylinder 8 by a nozzle 4. Generated combustion exhaust gas is discharged upward while exchanging heat with a water drum section 9, but one part of the gas functions as a recirculating flow, which passes through the outside of the combustion assisting cylinder 8 and enters to the inside from an opening section in the lower end section of the combustion assisting cylinder 8. A region formed between the combustion assisting cylinder 8 and an intermediate cylinder 2 functions as an evaporating region for evaporating fuel fed from the nozzle 4 and mixing it with air for combustion. The recirculating flow passing through the outside of the combustion assisting cylinder 8 exchanges heat with the water drum section 9 even in a recirculating process, but the quantity of heat is hardly exchanged in the recirculating process because the water drum section 9 is not mounted up to the bottom of the combustion chamber 1, the heat of vaporization required for the evaporating region can be fed, the evaporation of fuel is not delayed, and oil collecting on the bottom surface of the combustion chamber 1 is not changed into tar and does not deposit.