Abstract: A plurality of tubes (3) for flowing a heat transfer tedium, a plurality of fins (4), and tanks (1) connected to the tubes (3) is provisionally assembled. Each of the tubes (3) comprises a pair of formed plates (3a, 3b) coupled to each other. Each of the formed plates (3a, 3b) has a peripheral portion (20) and a plurality of protrusions (14) formed on an inside thereof and having through-holes in top ends thereof. The outer surface of the provisional assembly is coated with a noncorrosive flux (40) in an amount rate of 3 g/m.sup.2 or more on the base of the total area of the outer surfaces and inner surfaces of the tanks, fins and tubes. Then, the provisional assembly is subjected to a brazing treatment. The noncorrosive flux (40) is melted and flows to penetrate into the inside of the tubes (3) through the through-holes (45, 18a) to bond the paired formed plates to each other.