PURPOSE: To eliminate special heating means by a method wherein a catalyst is arranged at the upper part of atomization means with a space therebetween, an inverted-recessed part is made at a central part of the catalyst, while a flat part is made at a circumference thereof, and a plurality of fine through holes are made at the flat part.

CONSTITUTION: A space 10 is formed above a nozzle 4 and heat-resistant inorganic catalyst 9 is provided at an entire surface of the inside of the boiler 1. The catalyst 9 is provided with an inverted-recessed part 11 at its central part, with a flat part 12 and a plurality of fine holes 13 at the flat part 12 to make a honeybomb structure. Said central inverted-recessed part 11 has ceiling and oppositely faced against the nozzle 4. The catalyst 9 is made to support platium and paradium and the like as an oxidation catalyst. Thereby, it is made possible to transfer the catalyst 9 to a normal oxidation catalyst reaction after the catalyst is preheated to such a temperature as an oxidation catalyst reaction can be performed under a dispersion combustion at the time of ignition, resulting in that no special heating means is required.