A tube 101 is constituted by a pair of plates 111a, 111b which are fitted with each other in such a manner as to put an inner fin 101b between the plate 111a and the plate 111b. Differences in level 111c are formed on the second plate 111b, which fits inside, which differences in level each protrude inwardly by a distance equal to the thickness of the first plate 111a, whereby the outer wall surface of the tube 101 is made substantially level thereover. A gap which is formed between the outer wall surface of the tube 101 and a core plate, when the tube is passed through the core plate, can be as small as possible whereby the brazing properties can be improved.