To easily and surely position the insertion depth of an exit/entrance pipe in an exit/entrance hole in assembling the exit/entrance pipe to a tank of a heat exchanger.

A recessed part 9 is formed in at least a part of the outer circumference of an exit/entrance pipe 7 and a blazing filler material 10 is wound around the exit/entrance pipe 7 by matched to this recessed part 9. Secondarily, the exit/entrance pipe 7 is inserted into an exit/entrance hole 5 of a tank 4 till the blazing filler material 10 contacts with the circumferential edge of the exit/entrance hole of the tank. The insertion depth of the exit/ entrance pipe 7 can be thus easily and surely positioned by contacting of the blazing filler material 10 with the circumferential edge of the exit/entrance hole of the tank. The blazing filler material 10 is heated and the exit/entrance pipe 7 is blazed to the tank 4 of a heat exchanger 1 so that assembling of the heat exchanger of the exit/entrance pipe 7 to the tank is completed.