Abstract: A membrane filter comprises one or several tubular flow channels (1) for liquid which is to be treated in the filter. The walls of the flow channels consist on the inner side of membranes, which are supported by a supporting structure. An outer casing (2) surrounds the flow channels and limits a collection space (5) for permeate between the casing and the flow channels. According to the invention the flow channels (1) and the said collection space (5) are connected to separate outer circuits, comprising pumps to bring the liquid which is to be filtered (retentate) as well as permeate to circulate concurrently pass the membrane filter. The collection space is filled by filling bodies, which constitute an essential mechanical hinder for the flow of circulating permeate. Means in the form of disks (8) provided with holes, which abut against the outer casing, are arranged at the inlet to the flow channels.