A microporous modified-polytetrafluoroethylene membrane, being a microporous membrane produced by the steps of: producing a copolymer of hexafluoropropylene or perfluoroalkyl ether and tetrafluoroethylene having the number of moles not less than 50 times that of the hexafluoropropylene or perfluoroalkyl ether; forming the copolymer into the shape of a membrane; producing a nonporous fluororesin membrane by heating the membrane to the melting point of the copolymer or above to bake the copolymer; cooling the obtained nonporous fluororesin membrane; and subsequently stretching the nonporous fluororesin membrane; the microporous modified-polytetrafluoroethylene membrane having a mean flow pore diameter of 30 nm or less measured using a micropore distribution measuring instrument and by using propylene, 1,1,2,3,3,3-oxidised hexahydrofluoric acid as a liquid, the mean flow pore diameter obtained by a method in which the relationship between the pressure difference applied to the membrane and the rate of air flow passing through the membrane is measured both in the case where the membrane is dry, to form a dry curve, and where the membrane is wet with the liquid, to form a wet curve, the pressure difference at the intersection point of the wet curve and the curve drawn by halving the flow rate in the dry curve being denoted as P, the mean flow diameter being obtained by the following equation: mean flow pore diameter d = cγ / P, where c is a constant of 2,860 and γ is the surface tension of the liquid in dynes/cm and P is in Pa, wherein the tetrafluoroethylene used for forming the copolymer has the number of moles not more than 400 times that of the hexafluoropropylene or perfluoroalkyl ether. producing a copolymer of hexafluoropropylene or perfluoroalkyl ether and tetrafluoroethylene having the number of moles not less than 50 times that of the hexafluoropropylene or perfluoroalkyl ether; forming the copolymer into the shape of a membrane; producing a nonporous fluororesin membrane by heating the membrane to the melting point of the copolymer or above to bake the copolymer; cooling the obtained nonporous fluororesin membrane; and subsequently stretching the nonporous fluororesin membrane; the microporous modified-polytetrafluoroethylene membrane having a mean flow pore diameter of 30 nm or less measured using a micropore distribution measuring instrument and by using propylene, 1,1,2,3,3,3-oxidised hexahydrofluoric acid as a liquid, the mean flow pore diameter obtained by a method in which the relationship between the pressure difference applied to the membrane and the rate of air flow passing through the membrane is measured both in the case where the membrane is dry, to form a dry curve, and where the membrane is wet with the liquid, to form a wet curve, the pressure difference at the intersection point of the wet curve and the curve drawn by halving the flow rate in the dry curve being denoted as P, the mean flow diameter being obtained by the following equation: mean flow pore diameter d = cγ / P, where c is a constant of 2,860 and γ is the surface tension of the liquid in dynes/cm and P is in Pa, wherein the tetrafluoroethylene used for forming the copolymer has the number of moles not more than 400 times that of the hexafluoropropylene or perfluoroalkyl ether.