<p>Disclosed is a membrane separation device comprising a plurality of flat-membrane elements (52) arranged in parallel to form channels of liquid to be processed therebetween, and a collecting case (59) having a collecting space which is formed internally and communicates with the permeated liquid channel of the membrane element (52), wherein the collecting case (59) comprises a membrane element holding portion (53) having a plurality of slits into which the sides of respective membrane elements (52) are inserted, and a stop member (58) which abuts on the membrane element holding portion (53) in the collecting case (59), and a filler (55) for sealing between the membrane element (52) and the membrane element holding portion (53) watertightly is provided in the slit.</p>
