PURPOSE: To make it possible to remove a transmitted liquid flowline material, by providing folds to a semi-permeable membrane itself constituting a separated fluid flowline by spirally winding said membrane around a hollow pipe.

CONSTITUTION: Semi-permeable membranes 9a, 9b having folds 10a, 10b formed thereto are formed in an hermetically sealed and adhered state so as to communicate the interior of said membranes with small apertures of a hollow pipe and other three side ends are adhered in an envelope form. The adhered one is spirally wound around the hollow pipe to form a separated fluid flowline, that is, a transmitted liquid flowline. The folds 10a, 10b achieve the function of a transmitted liquid flowline material and the flowline material can be removed. Thus formed liquid separation element 1' is built in a cylindrical container 2 which is, in turn, hermetically sealed by side lids 3, 4. The raw liquid from a raw liquid supply port 5 is flowed into the container 2 toward the hollow pipe in a vortex form and flowed to the axial direction along the raw liquid flowline and discharged from a raw liquid discharge pipe 6. A part of the raw liquid transmits through the semi-permeable membranes and guided to the outside from a transmitted liquid discharge pipe 7 through the small apertures of the hollow pipe.