A heat exchanger A1 includes: a case 2 surrounding a heat transfer tube 4; and a guide member 5 for guiding heating gas entering the case 2 through an intake vent 21 to a first space region 24A and then to a second space region 24B, the guide member 5, together with a wall portion 20b, defining therebetween a gap 25b in communication with a gap 25a between the wall portion 20b and the heat transfer tube 4 for allowing part of the heating gas having been guided to the first space region 24A to proceed into the second space region 24B through the gaps 25a and 25b. This arrangement performs efficient heat recovery from the heating gas by effectively utilizing the entire heat transfer tube 4 while alleviating a problem that the case 2 is partially heated to elevated temperatures.