To provide a heat exchanger capable of efficiently recovering heat from a heating gas such as a combustion gas by further effectively utilizing the entire heat transfer tube in comparison with a conventional one, and minimizing disadvantage that a part of a case receiving the heat transfer tube is heated to a high temperature.

In this heat exchanger A1 in which the heating gas flowing into the case 2 from an air supply opening 21 of the case 2 surrounding the heat transfer tube 4, is moved toward a first space region 24A in the case 2 by guiding action of a guide member 5, and then moved to a second space region 24B, a gap 25b communicated with a gap 25a between a wall section 20b and the heat transfer tube 4 is formed between the guide member 5 and the wall section 20b, and a part of the heating gas guided to be moved to the first space region 24A by the guide member 5, can be moved to the second space region 24B through the gaps 25a, 25b.