A heat exchanger includes a second heat transfer tube disposed in a case on the upstream side of a first heat transfer tube and a plurality of fins in a heating gas flow direction and provided in contact with the inner surface of a side wall portion of the case, and a heat shield member. The heat shield member includes a tube cover portion that covers an outer peripheral surface inside portion of a part of the second heat transfer tube near the plurality of fins, and a first extension portion that extends from the tube cover portion toward the plurality of fins so as to close at least a part of a first gap formed between the second heat transfer tube and the plurality of fins. Thus, a problem whereby the second heat transfer tube corrodes easily can be eliminated, and the thermal efficiency of the heat exchanger can be improved.