Abstract: A fluid separation element comprising a casing and a bundle of selectively permeable hollow fibers accommodated therein, the bundle being potted with a resinous potting agent at the end portions and the gaps among the hollow fibers being blocked with the resinous potting agent, characterized in that (1) the ratio of the porosity of the membrane of the hollow fiber at the effective permeation portion (HA) and the porosity of the membrance of the hollow fiber at the potting portion (HB) satisfies the following condition: 0.9.gtoreq.HB/HA.gtoreq.0.4 and (2) the outer diameter of the hollow fiber at the effective permeation portion (OD.sub.A) and the outer diameter of the follow fiber at the potting portion (OD.sub.B) satisfies the following condition: 0.95.gtoreq.OD.sub.B /OD.sub.A .gtoreq.0.7, whereby the firm potting between the hollow fibers and the resinous potting agent is achieved, and a good separation efficiency without the increase of a pressure loss can be maintained for a long period of time.