PURPOSE: To obtain a heat exchanger having an increased heat exchange rate, by providing fins with projections so as to attain the division of a heat current and increase a heat transfer surface area.

CONSTITUTION: A high-temperature gas supplied from a high-temperature gas generating source, such as a burner is introduced from upstream ends of fins 12 into a heat exchanger. At this time, a part of the high-temperature gas is supplied as shown in streamlines 15 to a low-temperature portion on the downstream side of a pipe to attain excellent heat exchange. The other part of the high-temperature gas passes as shown in streamlines 16 opened portions 14 on the upstream ends of projections 13 to be subjected to heat exchange at the projections 13. The resulting gas passes parallel portions 18 of the fins 12 at the downstream side of the projections 13 to be discharged to the outside of the hat exchanger. Each of the projections 13 has an inclined portion 19 which serves to increase a heat transfer surface area. Owing to the dividion of heat current and an increase in heat transfer surfce area attained by the projections 13, a high heat exchange rate can be obtained.