A redox flow secondary battery comprising an electrolytic bath comprising: a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode; a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber, wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance; wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions; wherein the electrolyte membrane comprises an ion-exchange resin composition comprising 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, or an amine (NH 4, NH 3 R 1, NH 2 R 1 R 2, NHR 1 R 2 R 3, 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); and wherein the electrolyte membrane has an ion cluster diameter of 1.00 to 2.95 nm as measured in water at 25°C by a small angle X-ray method. a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode; a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber, wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance; wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions; wherein the electrolyte membrane comprises an ion-exchange resin composition comprising 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, or an amine (NH 4, NH 3 R 1, NH 2 R 1 R 2, NHR 1 R 2 R 3, 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); and wherein the electrolyte membrane has an ion cluster diameter of 1.00 to 2.95 nm as measured in water at 25°C by a small angle X-ray method.