Abstract: A filter for cross-flow filtration of a fluid containing suspended or emulsified substance is constructed of equally dimensioned plates. Between the plates there are passages for retentate and permeate flow. The plates have in- and outlet openings to form collection channels, through which unfiltered fluid is led to and retentate and permeate is led away from the passages. There are plates (1) (membrane plates) comprising a membrane of material which is chemically and physically inert to the fluid, which membrane has a pore size of 0.1-50 .mu.m through which membrane the permeate passes. There are also plates (2) which are impervious to the fluid. Retentate (6) and permeate (8) passages are formed between the plates (1, 2), which all have three openings (3, 4, 5). The retentate passages (6) are connected to the channel for unfiltered fluid and to the channel for retentate and sealed against the channel for permeate.