Method of filtration using porous membranes

A method of filtration comprising:

• a filtration in which a liquid to be filtered is filtered through a porous membrane formed from a resin, and cleaning an inside of the porous membrane after the filtration,
• wherein the porous membrane has, in all visual fields in a cross section of the inside of the membrane, an area proportion of a resin portion having an area of 0.1 μm2 or more and 1 μm2 or less in each of the visual fields, which is 70% or more relative to a total area of all resin portions included in each of the visual fields, and an area proportion of a resin portion having an area of more than 1 μm2 and less than 10 μm2 in each of the visual fields, which is 2% to 30% relative to the total area of all the resin portions included in each of the visual fields, and an area proportion of a resin portion having an area of 10 μm2 or more in each of the visual fields, which is 15% or less relative to the total area of all the resin portions included in each of the visual fields, and the porous membrane is formed from a fluorine resin of a polymer selected from a group consisting of vinylidene fluoride resin (PVDF), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), and ethylene-tetrafluoroethylene copolymer (ETFE), and
• the porous membrane is used in the cleaning comprising passing a 4% or more sodium hydroxide aqueous solution through the porous membrane,
• wherein a relationship between an elongation E0 of the porous membrane before the cleaning and an elongation E10 of the porous membrane after the cleaning is E10/E0×100≥70%, and
• wherein a relationship between a flux L0 of the porous membrane at a start of the cleaning and a flux L10 of the porous membrane after the cleaning is 105≥% L10/L0×100≥95%.