Use of an electrolyte membrane in a redox flow secondary battery, the electrolyte membrane comprising an ion-exchange resin composition comprising a polyelectrolyte polymer as a main component, and having a fluorine-based microporous membrane as a reinforcing material; wherein the polyelectrolyte polymer is a fluorine-based polyelectrolyte polymer having a structure represented by the following formula: -[CF 2 CX 1 X 2] a -[CF 2 -CF((-O-CF 2 -CF(CF 2 X 3)) b -O c -(CFR 1) d -(CFR 2) e -(CF 2) f -X 4)] g - wherein X 1, X 2, and X 3 each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X 4 represents COOZ, SO 3 Z, PO 3 Z 2, or PO 3 HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, NH 4, NH 3 R 1, NH 2 R 1 R 2, NHR 1 R 2 R 3, or NR 1 R 2 R 3 R 4; wherein R 1, R 2, R 3, and R 4 each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X 4 is PO 3 Z 2, Z may be identical or different; R 1 and R 2 each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0 ≤ a < 1, 0 < g ≤ 1, and a + g = 1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6, with the proviso that d, e, and f are not 0 at the same time; wherein the fluorine-based microporous membrane has a structure in which the fluorine-based microporous membrane is impregnated with the polyelectrolyte polymer, and 90% or more of the internal volume of the fluorine-based microporous membrane is occluded; and in the pore distribution of the fluorine-based microporous membrane, the ratio of the number of pores whose pore diameters are 0.08 μm to 5.0 μm to the total number of pores of the microporous membrane, as measured in the pore diameter measurement range of 0.065 μm to 10.0 μm by a bubble point half dry method using the bubble point method described in JIS-K-3832, of the fluorine-based microporous membrane is 0.5 or higher. wherein X 1, X 2, and X 3 each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X 4 represents COOZ, SO 3 Z, PO 3 Z 2, or PO 3 HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, NH 4, NH 3 R 1, NH 2 R 1 R 2, NHR 1 R 2 R 3, or NR 1 R 2 R 3 R 4; wherein R 1, R 2, R 3, and R 4 each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X 4 is PO 3 Z 2, Z may be identical or different; R 1 and R 2 each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0 ≤ a < 1, 0 < g ≤ 1, and a + g = 1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6, with the proviso that d, e, and f are not 0 at the same time; wherein the fluorine-based microporous membrane has a structure in which the fluorine-based microporous membrane is impregnated with the polyelectrolyte polymer, and 90% or more of the internal volume of the fluorine-based microporous membrane is occluded; and in the pore distribution of the fluorine-based microporous membrane, the ratio of the number of pores whose pore diameters are 0.08 μm to 5.0 μm to the total number of pores of the microporous membrane, as measured in the pore diameter measurement range of 0.065 μm to 10.0 μm by a bubble point half dry method using the bubble point method described in JIS-K-3832, of the fluorine-based microporous membrane is 0.5 or higher.