PURPOSE: To make easy the replacement of the membrane without damaging the semipermeable membrane of the separator, by inserting the expanded separator element into the pressure pipe.

CONSTITUTION: The pressure pipe 2 is inserted into the liquid collecting pipe 3, and both ends are connected to the joint 4. Next, the end part of the separator element 1, whose both ends are expanded, is slightly deformed inward, and is pushed into the pressure pipe 2 from the side of one joint 4, and both ends of the separator element 1 are fitted to the taper of the joint 4. The deformed end part of the separator element 1 is restored to the initial expanded condition. Finally, the pipe 6 is connected,and the capped nut 62 is fastened until the sealing mechanism is completed. When the separator element 1 is to be detached, the procedure is inversed. Furthermore, the pressure pipe 2 is made longer than the liquid collecting pipe 3, and the permeated liquid outlet 42 is installed between the liquid collecting pipe 3 on the side plane of the joint 4 and the pressure pipe 2. According to this mechanism, any special tool is unnecessary for replacing the membrane.