A method for producing a separator for an electricity storage device, comprising the following steps: a sheet-forming step in which a silane graft-modified polyolefin, polyethylene and a plasticizer are extruded into a sheet using an extruder, cooled to solidification and shaped into a molded sheet; a stretching step in which the molded sheet is subjected to biaxial stretching to a 20-fold to 250-fold area increase to form a stretched sheet; a porous body-forming step in which the plasticizer is extracted from the stretched sheet to form a porous body; a heat treatment step in which the porous body is subjected to heat treatment and subjected to stretching and relaxation in the transverse direction to obtain a heat-treated porous body; an affinity treatment step in which the heat-treated porous body is immersed in an organic solvent that is amphiphilic for water and organic materials, and the liquid affinity of the heat-treated porous body is increased to obtain an affinity-treated porous body impregnated with the organic solvent; a crosslinking step in which the affinity-treated porous body is contacted with a mixture of an organometallic catalyst and water, or is immersed in a base solution or an acid solution and subjected to silane dehydration condensation reaction to form oligosiloxane bonds, obtaining a crosslinked porous body; and a washing and drying step in which the crosslinked porous body is washed with water and dried, wherein the silane-modified polyolefin has a structure with a polyolefin as the main chain and alkoxysilyl groups grafted onto the main chain, and wherein the organic solvent is selected from the group consisting of alcohols, acetone, ethylene carbonate, N-methyl-2-pyrrolidone, and dimethyl sulfoxide. a sheet-forming step in which a silane graft-modified polyolefin, polyethylene and a plasticizer are extruded into a sheet using an extruder, cooled to solidification and shaped into a molded sheet; a stretching step in which the molded sheet is subjected to biaxial stretching to a 20-fold to 250-fold area increase to form a stretched sheet; a porous body-forming step in which the plasticizer is extracted from the stretched sheet to form a porous body; a heat treatment step in which the porous body is subjected to heat treatment and subjected to stretching and relaxation in the transverse direction to obtain a heat-treated porous body; an affinity treatment step in which the heat-treated porous body is immersed in an organic solvent that is amphiphilic for water and organic materials, and the liquid affinity of the heat-treated porous body is increased to obtain an affinity-treated porous body impregnated with the organic solvent; a crosslinking step in which the affinity-treated porous body is contacted with a mixture of an organometallic catalyst and water, or is immersed in a base solution or an acid solution and subjected to silane dehydration condensation reaction to form oligosiloxane bonds, obtaining a crosslinked porous body; and a washing and drying step in which the crosslinked porous body is washed with water and dried, wherein the silane-modified polyolefin has a structure with a polyolefin as the main chain and alkoxysilyl groups grafted onto the main chain, and wherein the organic solvent is selected from the group consisting of alcohols, acetone, ethylene carbonate, N-methyl-2-pyrrolidone, and dimethyl sulfoxide.