To prevent a porous mat from being clogged, and at the same time, facilitate the maintenance by a method wherein air which is taken in from an air inlet is introduced to the porous mat while coming into contact with the surface of water which is filled in a recessed part.

Air is introduced by operating a blower fan 4, and at the same time, a combustion gas is discharged from a feeding pipe 5, and the mixed gas is fed to a combustion burner part 1, and at the same time, by an igniting device, a surface combustion is performed by a porous mat 7. Then, by making water in a water pipe 8 flow, a heatexchange is performed at a heat-exchanging part 6, and hot water is obtained. In this case, the air which is introduced from an air inlet part 3, meanders by baffle plates 10a, 10b, and comes into contact with water which is filled in a recessed part 2a by a plurality of times, and dust and dirt, etc., are removed, and then, is fed to the combustion burner part 1 together with the combustion gas. Also, by using the porous mat 7 wherein a metal fine wire of which the diameter is 100 μm or less is formed into a mat-shape, a uniform combustion can be performed.