complete combustion while passing through the catalytic mat 2 therefore since activity of the catalyst is reduced due to lowering of the temperature combustion heat is retained by the diffusion heat insulation layer 3 and heat insulation diffusion layer 12 fior uniform diffusion however owing to the fact that the peripheral portion of the catalytic mat mat 2 contacts the flange portion 6a etc of the casing 6 and is inevitably more readily cooled than the central portion thereof such peripheral portion contacting the casing 6 is given an increased density for blocking flow of the combustion fuel while direct contact of the catalytic mat 2 with such flange portion is avoided as far as possible the heat insulation layer 12 with the high density has for its main object to insulate against loss of heat and has a thickness approximately 1 2 that of the diffusion heat insulation layer 3 with a density about two times that of said layer 3 while said heat insulation layer 3 is mainly intended for uniform gas diffusion as is seen from the foregoing description in the construction of the catalytic mat layer portion according to the embodiment of the present invention as explained so far owing to the various layers provided therein uniform diffusion of combustion fuel is advantageously achieved without the the catalytic mat 2 being cooled by the casing 6 moreover since the contacting portion of the catalytic mat 2 is arranged to be high in density and small in thickness not only is erroneous work eliminated during assembly but also the catalytic mat 2 may be efficiently utilized without being cooled excessively particularly for appliances used during cold winters such as gas heaters and.