PURPOSE: To effectively strip deposits from a filtration film by providing plural original liquid inflow ports connected to a selector valve to a film separator, generating swirling flow on the surface of the filtration film by the original liquid from the prescribed original liquid inflow port and further intermittently turning over the swirling flow by the selection of the selector valve.

CONSTITUTION: The plural original liquid inflow ports 71W74 for supplying the original liquid in a tangent direction are formed to the outside peripheral side wall of the cylindrical film separator 4 disposed with the circular filtration film 3. The clockwise swirling flow is first generated near the surface of the filtration film 3 when the original liquid is supplied through the original liquid inflow ports 71, 72 connected to branch pipes 9a, 9b by setting the original liquid supply selector valve 8 to the 1st piping 9 side. The counterclockwise swirling flow is then generated when the original liquid is supplied through the original liquid inflow ports 73, 74 connected to branch pipes 10a, 10b by changing over the valve 8 to the 2nd piping 10 side. The deposits of the filtration film 3 are scrapped off by intermittently and alternately executing the above-mentioned operations. Clogging is thus prevented.