An exothermic apparatus uses heat generated by the combustion of gas fuel or liquid fuel. The exothermic apparatus comprises a mixing section in which fuel gas and air are mixed with each other; and a casing disposed downstream of the mixing section and accommodating a combustion chamber. A fin provided in the combustion chamber is substantially parallel with the flow direction of mixed gas. A catalyst layer is in close contact with an inner surface of the combustion chamber and an outer surface of the fin. In this manner, fuel gas is efficiently brought in contact with the surface of the catalyst layer and the temperature of the catalyst is prevented from fluctuating even though heating quantity in an exothermic section changes. Thus, catalytic combustion can be reliably accomplished in the compact combustion chamber.
