To impart desire strength to a heat exchanger by enhancing the machining accuracy of dimple tube in the manufacturing process of the heat exchanger thereby reducing error of manufacture.

Raised parts 25 are formed on a flat plate 20 clad with soldering material and the flat plate 20 is bent to form a flat tube 11 wherein corresponding raised parts 25 abut against each other at the top part thereof. End part of the tube 11 is then inserted into each of a plurality of tube insertion holes 36 made in headers 12, 13, cooling fins 14 are provided between a plurality of tubes 11, 11 fitted to the headers 12, 13 and then the cooling fins 14 are heated in order to solder each abutting part. In such a process for manufacturing a heat exchanger, a raised part 25' closest to the end part of the tube 11 is elongated in the lengthwise direction and formed lager than other raised parts 25.