To improve the insertability of the inserting part of a tube and the flow-in and the flow-out of a medium by deforming the tube so as to increase the height size and absorbing the excess thickness part into this deformed part, at the time of forming a flattening part for inserting the tube at the end part of the tube.

For example, a single aluminum plate stack is formed to a flat-shaped and also, plural lines of beads 11, 11 are formed over the longitudinal direction of the tube stock to form the tube 2 for heat exchanger dividing the inner part of the tube into plural medium flowing passages 12, 12 with these beads 11, 11. The position inserting the end part of this tube 2 into a header pipe is formed to push back as the flay surface state so as to use as the inserting part 2 of the tube, and the height size of the inserting part 2a of the tube is increased to such degree as not to develop at least the excess thickness part of the tube. The end part of the tube 2 is inserted into the inserting hole 5 of the tube in the header pipe 4 formed according to the outer shape of the inserting part 2a of the tube.