To reduce man-hours required for inserting an inner fin and to improve mass production property by inserting a recessed curved part for connection into the gap between the butt part of a tube body and a surface center and arranging an inner fin at both the sides of the butt part.

A tube body 2 is bent in a width direction, and both the end parts are bent and formed at the side of an inner surface and are subjected to butting on an opposing flat surface, thus forming a butt part 1. Then, a gap 4 is formed between the tip part of the butt part 1 and the center of the inside of the tube. Then, a corrugated inner fin 3 that is inserted into the tube body 2 is bent and formed in a corrugation with a fixed pitch. At the same time, a recessed part 5 for connection with a pitch that is different from other corrugation pitches is formed at a central position in the advance direction of the wave. The corrugated inner fin 3 is entirely inserted into the tube body 2 so that the recessed part 5 for connection can be inserted into the gap 4. Then, the corrugated inner fin 3 is arranged essentially equally at both the ends with the butt part 1 as a boundary.