A filtering method which prevents adhesion between an outlet vessel and a filter element and which uses a flexible filter which allows smooth flow of liquid and is superior in filtering performance; and a filtering system. A filtering method which uses a flexible vessel having an inlet and an outlet for liquid, a sheet-like filter element, and a filter in which the inlet and outlet for liquid are separated by the filter element, wherein in filtering a liquid by the action of gravity or of a liquid feed pump, the pressure in the outlet of the filter is not less than 0 mmHg with respect to the atmospheric pressure; and a filtering system. Further, a filtering method in which a suitable combination of the filter and the hydrodynamic characteristics of the flow channel upstream or downstream of the filter is selected to make the pressure in the outlet of the filter not less than 0 mmHg with respect to the atmospheric pressure; and a filtering system.