PURPOSE: To easily form double-pipe construction without damaging a separation membrane tube by inserting a flanged sleeve into each of connection ends of a separation membrane tube and a solvent pipe, and making the end faces of flanged parts of both sleeves stick mutually with exerted pressure.

CONSTITUTION: A pair of joints 25 on each of which a sample water pipe end connection 21, a solvent pipe end connection 22 and an outer pipe end connection 23 are provided in three ways is arranged so that the outer pipe end connections 23 of both joints 25 are connected by an outer pipe 27. On the other hand, a sample water pipe 30 is connected to the sample water pipe end connection 21 of each of the joints 25 and a solvent pipe 34 is connected to the solvent pipe end connection 22. Further, a separation membrane tube 35 is inserted into the outer pipe 27 and both ends of the separation membrane tube 35 are connected to the solvent pipes 34, respectively. A connection 37 of the solvent pipe 34 and the separation membrane tube 35 consists of flanged sleeves 38, 39 inserted into the connection end of the separation membrane tube 35 and the connection end of the solvent pipe 34, respectively. The flanged parts 40, 41 are made to stick together by contacting the end faces of the flanged parts with each other followed by pressurization.