PURPOSE: To permit the assembly of the core without deforming radiating fins by a method wherein both side surfaces of a heat exchanging tube are inflated beforehand and are compressed upon forming the core by the amount of the inflation.

CONSTITUTION: The fore end 2a of the radiating fin 2 is inserted between the heat exchanging tubes 1 and is pressed between them by imposing a pushing force toward the aft side of the tubes as shown by the arrow signs. In this case, both sides of the radiating fin 2 are not readily deformed by an edge effect while, on the contrary, the heat exchanging tubes become intimate with the edges of the fin readily in accordance with the pushing forces, since the configurations of the heat exchanging tubes are closed to flat plates. Thus, the top 2a of the radiating fin 2 is adhered to the side surfaces 1b of the heat exchanging tubes 1, therefore, if the adhered parts thereof are secured by soldering, the core may be assembled without deforming the radiating fin 2.