To provide a filtering method capable of improving filtering ability while maintaining the reliability of filtering and reducing the burdens of the maintenance work of a device, and its device.

The method for filtering activated sludge water Ws by using a cylindrical separation film body 31 installed inside a treating water tank 2 is disclosed. While the activated sludge water Ws is pressurized and circulated inside the cylindrical separation film body 31, treating water Wf is made to flow out from the inside of a cylindrical film cartridge 30 to the outside by the positive pressure P1 of the activated sludge water Ws and filtering is executed.