PURPOSE: To minimize the amount of material without sacrificing the strength of a return bent part by making the wall thickness of a thinly elongated tubular member with fins bent in a serpentine pattern greater at the return bend parts than at cross parts.

CONSTITUTION: In the formation of heat exchange elements 18, the portions 31 and 31a of upper and lower surfaces 30 and 30a of a tubular member 19 are slightly compressed before the fins are cut. Fins 21-23 are vertically elongated greater than the fins 24-26. This is because that a rib part into which the fins 24-26 and 24a-26a are cut becomes thinner in the groups of the fins by compressing the tubular member 19 in the surrounding area of the return bend parts 42, 42a, 42b and 42c. The surface parts 31 and 31a thus compressed are arranged on the periphery of the return bend parts of the heat exchange element 18 and in the rib parts 27-29 and 27a-29b, the upper and lower surfaces 30 and 30a are pressed inward to make the wall thickness thereof larger over the length equal to the linearly converted length of the return bend parts 42, 42a, 42b and 42c.