Abstract: A pipe (40) having a specific length is filled with nitrogen gas (4a). From opening ends of connecting portions (42, 42) located at respective ends of the pipe (40), cross sectionally hat-shaped block plugs (50) made of brazing material are pressed into the connecting portions (42, 42) respectively and are fitted thereto for the purpose of sealing the pipe (40). Next, one end of a connecting piece (60) is inserted into the connecting portion (42) of one pipe (40) so as to break through the block plug (50) fitted thereto. The other end of the connecting piece (60) is also inserted into the connecting portion (42) of another pipe (40) so as to break through the block plug (50) fitted thereto. Thereafter, the block plugs (50) are melt so that the connecting portions (42) of both the pipes (40) are brazed with the connecting piece (60).