PURPOSE: To prevent a first semipermeable membrane from being damaged by the backward pressure of a permeated liq. by obviating the dipping of the first semipermeable membrane between two semipermeable hollow fibers in the permeated liq.

CONSTITUTION: A cap 8 having a fluid inlet 5 and a cap 8' having a fluid outlet 6 are respectively engaged with both ends of the outer cylinder 4 provided with an air vent 7. Both ends of semipermeable hollow fibers 1 and 2 in the outer cylinder 4 are fixed in the outer cylinder 4 by fixing members 3 and 3'. The hollow fibers 1 and 2 are arranged in parallel with the inner wall of the outer cylinder 4, one end of the first semipermeable hollow fiber 1 is sealed in the fixing member 3', and the other end is fixed by the fixing member 3, kept open and communicated with the fluid inlet 5. Meanwhile, one end of the second semipermeable hollow fiber 2 is sealed in the fixing member 3, and the other end is fixed by the fixing member 3', kept open and communicated with the fluid outlet 6. The first semipermeable hollow fiber 1 is made to be never dipped in the permeated liq. Consequently, the semipermeable hollow fiber 1 as the first semipermeable membrane is not damaged.