To contrive filtration and filter membrane washing at the same time by providing an original liquid inflow chamber, a filter chamber provided multiply concentrically with plural tubular filter membranes made of metal connected to an original liquid outflow chamber and having discharge ports for filtered liquid, and a means for reciprocating a part of brushes for cleaning the inner peripheral surface of the filter membranes in the axial direction thereof.

Original liquid flowing into an inflow cylinder 2 from an original liquid inflow port 21 permeates through filter membranes 3 by inflow pressure and flows out to between an outer cylinder 10 and the filter membranes 3, and the filtered liquid is discharged to outside the device from discharge ports 14. Original liquid containing solid matter or the like which does not permeate through the filter membranes 3 flows into a circulating cylinder 3 and is returned to an original liquid storage tank from an outflow port 31. In the case filtration is performed for a long period of time and contaminants are stuck on the inner surface of some filter membranes 13 to obtain no prescribed permeation flux, shafts 15 are vertically moved by driving a cam member 34, allowing brushes 16 to slide on the inner surface of the filter membranes 13 to brush off the contaminants stuck thereon. As a result, the filter membranes are washed while a filtration process is performed to attain high filtration capacity.