To ensure pressure resistance of a heat transfer pipe by ensuring the strength of brazing between an inner fin and an inner peripheral surface of the heat transfer pipe.

Holder parts 11a, 11a are formed on an outer peripheral surface of bent parts 10, 10, part of an inner fin 9a, that make contact with an inner peripheral surface of a heat transfer pipe 3. The flux held in each holder part 11a, 11a is prevented from being shaved off when the inner fin 9a is inserted into the heat transfer pipe 3. Accordingly, the amount of the flux is ensured which existent at a contact part between the inner fin 9a and the inner peripheral surface of the heat transfer pipe 3, and hence the strength of brazing is ensured.