PURPOSE: To permit to effect heat exchange between air, flowing between fin elements, and the fin efficiently by a method wherein a multitude of fin elements, consisting of metallic thin plates, whose both end rims are bent to form base plate sections, abutting sections and piling and supporting sections, are piled along the lengthwise direction of heat transfer tubes.

CONSTITUTION: The fin element 8, formed by bending the metallic thin plate, is formed with the recessed base plate section 9 at the center of the upper surface thereof, the abutting sections 10, 10 formed by bending both end rims of the metallic thin plate vertically and downwardly, and the piling and supporting sections 11 formed by bending the end rim parts of the abutting sections 10, 10 vertically and inwardly. Further, louvers 12, cut-up pieces 13, protuberances 14, bend-up parts 15 and bend-up parts 16 are formed in the element 8. The fins 2 are made by piling a multitude of fin elements 8, thus constituted, to constitute a core section together with the heat transfer tubes 1, 1. In this case, respective fin elements 8 are brazed mutually while respective fin elements 8 and the heat transfer tubes 1, 1 are brazed to connect them.