PURPOSE: To attain filtration of high solid suspension by successively laminating a secondary side liquid passing sheet, fine filtering film, primary side net type sheet and back pressure preventing frame on both sides of film supporting body, respectively, and providing entrance of fluid and a sealing means of the filter housing on to the side part of the relevant laminated body.

CONSTITUTION: Original liquid for filtration is entered into the filtration room 12 from a primary entrance 13, and collected into a liquid collecting part 11 once after being filtratered and discharged from the secondary entrance 14. Before filtration is interrupted and back washing is started, or before back washing is completed and filtration is restarted, the liquid remaining in the filtration room 12 is discharged from the second side entrance 14 by pressurized gas from entrance 16 of pressurized gas. And, before start of filtration, air in the filtration room is excluded from air vent hole 15. Original liquid entered into the filtration room 12 from the primary side hole 17 is passed through the primary side net type sheet 26 of the film element 20, fine filtration film 25, second side liquid passing sheet 24, and film supporting body 23 and collected into the liquid collecting part 11 through the filtrate discharge hole 28. Back washing solution is moved in the direction entirely reverse to direction of the filtrate.