PURPOSE: To manufacture a membrane separation unit which is excellent in solvent resistance and chemical resistance, etc., and inexpensive, and also, small in size by preparing a highly concentrated liquid suspension of a sealant consisting of a resin that has a lower melting point as compared with the resin used as the constituent material of hollow fiber membranes, thereafter immersing bound ends of the hollow fiber membranes in the suspension, then heating the bound ends while controlling the temp. and thereafter solidifying the ends.

CONSTITUTION: In this manufacture, a fluidizing liquid is added to fine powder of a second olefinic resin that is used as the constituent material of a sealant 2 and has a melting point in a lower temp. region than the melting point of a first olefinic resin used as the constituent material of hollow fiber membranes 1 to fluidize the fine powder and to prepare a highly concentrated liquid suspension. Then, the hollow fiber membranes 1 are bound into a U-shaped bundle and the bound ends are immersed in this highly concentrated liquid suspension and heated to a temp. lower than the melting point of the first olefinic resin and higher than the melting point of the second olefinic resin. At this time, the liquid is evaporated by the fluidization of the fine powder and the sealant 2 is converted into a melted and flowable state. Thereafter, the bound ends are slowly cooled to ordinary temp. to solidify them. Thus, the space between every adjacent two of the hollow fiber membranes 1 at each of the bound ends is sealed.