A heat exchanger includes a heat transfer tube in which a plurality of tube body portions are arranged in a two-tier staggered pattern in a flow direction of heating gas, and a fin, wherein a plurality of cut-out recessed portions are provided in the fin by cutting out sites between first and second upstream side tube body portions, and each cut-out recessed portion extends toward a downstream side in the flow direction of the heating gas beyond the site between the first and second upstream side tube body portions so as to cross straight lines linking a center of a first downstream side tube body portion, which is positioned between the first and second upstream side tube body portions in the width direction, and respective centers of the first and second upstream side tube body portions. Thus, the temperature distribution of the fin is made even, thereby suppressing the generation of thermal stress, and as a result, distortion of the heat transfer tube is suppressed.