PURPOSE: To braze a tube end easily and surely to each hole of a header plate in a heat exchanger.

CONSTITUTION: A bundle of parallel tubes 26, arranged in groups have tube ends fitted, respectively in a plurality of holes 22 made in the header plate 10 of heat exchanger and brazed thereto. Each hole 22 of the header plate 10 has a slender cross section to be fitted with mutually touching tube ends in a group, so that the long axes of the cross section of the tubes 26 are mutually aligned. The slender cross section of each hole 22 has a long axis AA and the length L thereof, which before the tube 26 is inserted into the hole 22, is shorter than the sum of the initial length of the long axis of the cross section of the tube ends 24.