PURPOSE: To improve corrosion resistance by bringing a brazing material of an inner skin material into contact with that of a multi-skin material through a polygonal hole or slit at the time of brazing, and using as a fin material aluminum or its alloy having a baser potential than that of a core material of a core plate.

CONSTITUTION: Of brazing material joints of two types of a brazing material joint 15 connected with an inner skin material 12 and a brazing material joint 14 connected with an outer skin material 11, the former is formed by integrating a nobler brazing material of an inner surface. Accordingly, since its potential is nobler than that of a core material 10, the material 12 of the joint 15 is protected in a sacrificially corrosive manner for the material 10 and no corrosion to penetration occurs. A potential of a brazing material 24 is affected by influences of both the materials 11, 12 at the other joint 14 to becomes substantially the same level as the potential of the material 10. That is, the joint 14 is formed mostly by integrating the material 11 of the baser outer surface, and hence the material 24 is affected by influence of the material 11. Thus, the joint 14 is not penetrated by corrosion.