PURPOSE: To improve a reignition temperature of a catalyst and to efficiently convert reaction heat of a heat source apparatus by increasing a noble metal carriage of a catalyst disposed on a lower surface side of a combustion chamber as compared with a catalyst disposed at an upper surface side of the chamber.

CONSTITUTION: A steam generating chamber 9 is provided on an upper surface of a mold vessel 7 corresponding to an upper part of an outer wall of a combustion chamber 6, and a steam channel chamber 13 and an exhaust gas channel chamber 14 are provided on a lower surface of the outer wall of the chamber 6. Heat transfer fins 15a, 15b are provided on the wall of the chamber 6. Catalysts 16, 17 are oppositely fixedly installed on inner walls of the vessels 7, 8, and a piezoelectric fire element is disposed at the rear of the catalysts 16, 17. A catalyst coating layer of the catalyst 16 is formed on a surface of a base material by forming an oxide film on the surface of the material having three recesses, then dipping it in catalytic slurry and heat treating it. A catalyst coating layer of the catalyst 17 is formed similarly on the surface of the material having the recesses in a shape engaged with the catalyst 16 in such a manner that noble metal carriage amount is increased as compared with that of the catalyst 16.