A method for producing a microporous film, the method comprising: melt-molding a resin composition comprising a resin component comprising polypropylene, from 0.01 to 20 parts by weight of a chemically modified cellulose or chemically modified cellulose nanofiber per 100 parts by weight of the resin component, and from 0.01 to 3 parts by weight of a β-crystal nucleating agent per 100 parts by weight of the resin component, into a film-like material in which a crystal phase of a polypropylene component in the resin composition has a β-crystal forming ability of 60% or more; and subsequently stretching the film-like material at a temperature of 60 to 160°C; wherein the β-crystal nucleating agent is any one of: at least one member selected from the group consisting of N,N'-diphenylhexanediamide, N,N'-dicyclohexylterephthalamide and N,N'-dicyclohexyl-2,6-naphthalenedicarboxam ide; at least one member selected from the group consisting of N,N'-dicyclohexanecarbonyl-p-phenylenediamine, N,N'-dibenzoyl-1,5-diaminonaphthalene, N,N'-dibenzoyl-1,4-diaminocyclohexane and N,N'-dicyclohexanecarbonyl-1,4-diaminocyclohexane; at least one member selected from the group consisting of N-cyclohexyl-4-(N-cyclohexanecarbonylamino)benzamide and N-phenyl-5-(N-benzoylamino)pentanamide; and a mixture of two or more amide-based compounds in to; and wherein the chemically modified cellulose or chemically modified cellulose nanofiber is a chemically modified cellulose or chemically modified cellulose nanofiber prepared by adding from 2 to 10 wt% of a dibasic acid anhydride by kneading and reaction at 80 to 140°C, followed by addition-polymerizing from 1 to 20 wt% of an alkylene oxide under heating at 80 to 140°C in a pressure-resistant container. melt-molding a resin composition comprising a resin component comprising polypropylene, from 0.01 to 20 parts by weight of a chemically modified cellulose or chemically modified cellulose nanofiber per 100 parts by weight of the resin component, and from 0.01 to 3 parts by weight of a β-crystal nucleating agent per 100 parts by weight of the resin component, into a film-like material in which a crystal phase of a polypropylene component in the resin composition has a β-crystal forming ability of 60% or more; and subsequently stretching the film-like material at a temperature of 60 to 160°C; wherein the β-crystal nucleating agent is any one of: at least one member selected from the group consisting of N,N'-diphenylhexanediamide, N,N'-dicyclohexylterephthalamide and N,N'-dicyclohexyl-2,6-naphthalenedicarboxam ide; at least one member selected from the group consisting of N,N'-dicyclohexanecarbonyl-p-phenylenediamine, N,N'-dibenzoyl-1,5-diaminonaphthalene, N,N'-dibenzoyl-1,4-diaminocyclohexane and N,N'-dicyclohexanecarbonyl-1,4-diaminocyclohexane; at least one member selected from the group consisting of N-cyclohexyl-4-(N-cyclohexanecarbonylamino)benzamide and N-phenyl-5-(N-benzoylamino)pentanamide; and at least one member selected from the group consisting of N,N'-diphenylhexanediamide, N,N'-dicyclohexylterephthalamide and N,N'-dicyclohexyl-2,6-naphthalenedicarboxam ide; at least one member selected from the group consisting of N,N'-dicyclohexanecarbonyl-p-phenylenediamine, N,N'-dibenzoyl-1,5-diaminonaphthalene, N,N'-dibenzoyl-1,4-diaminocyclohexane and N,N'-dicyclohexanecarbonyl-1,4-diaminocyclohexane; at least one member selected from the group consisting of N-cyclohexyl-4-(N-cyclohexanecarbonylamino)benzamide and N-phenyl-5-(N-benzoylamino)pentanamide; and a mixture of two or more amide-based compounds in to; and wherein the chemically modified cellulose or chemically modified cellulose nanofiber is a chemically modified cellulose or chemically modified cellulose nanofiber prepared by adding from 2 to 10 wt% of a dibasic acid anhydride by kneading and reaction at 80 to 140°C, followed by addition-polymerizing from 1 to 20 wt% of an alkylene oxide under heating at 80 to 140°C in a pressure-resistant container.