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: An assemblage, pinching the radiating fin 2 between the neighboring heat exchanging tubes 1, is arranged in left-and-right direction to constitute the core, and pressing forces are imposed from the left and right end surfaces of the core. At the contacting surfaces thereof, the radiating fin 2 functions to push the side faces of the heat exchanging tubes 1. Whereby, the side surfaces 1b of the heat exchanging tubes 1 are pushed by the radiating fin 2, deformed so as to narrow the width of the tube, the tops 2a of the radiating fin 2 are adhered to the side surfaces of the heat exchanging tubes 1 which were brought into parallel surfaces and thus the core may be assembled without deforming the radiating fin 2.