Abstract: Low melting point metal such as Zn is sprayed onto a flat surface (3a) of an aluminum flat tube (3) to form a sprayed metal coating (15) thereon inmediately after extrusion of the flat tube. Inmediately thereafter, a brazing foil (11) is integrally adhered onto the sprayed metal coating (15). The obtained flat tube (3) with the brazing foil is combined with a corrugated fin (4) to form a temporary assembly. Then, the temporary assembly is heated at a predetermined temperature to braze the tube (3) and the fin (4) to thereby form a heat exchanger core. The low melting point metal may be sprayed on both the flat surface (3a) of the flat tube and the surface of the brazing foil (11). Thus, it becomes possible to obtain a lightweight thin fin and to decrease material costs.