A heat exchanger includes fins (111) having a plurality of sets of two louvres (111c) substantially triangular in shape which are formed in such a manner that the distance from a flat plate portion (111a) increases progressively downstream in the gas flow direction. The two louvres (111c) of each set are arranged inwardly slanted along the gas flow direction, so that a vertical swirl is generated to draw the gas flow between the louvres (111c). The gas flowing in the vicinity of the flat plate portion (111a) and the gas flowing in the vicinity of a vertical plate portion (111b) are thus accelerated. As a result, the heat conductivity of the gas and the fins (111) is improved and the particulate matter attached on the surface of the fins (111) can be blown off, thereby preventing the clogging of the fins (111).