NOVELTY - A redox-flow secondary battery (10) comprises electrolytic cell (6) having positive electrode cell chamber (2) containing positive electrode (1) including carbon electrode, and positive electrode electrolyte solution, negative electrode cell chamber (4) containing negative electrode (3) including carbon electrode, and negative electrode electrolyte solution, and electrolytic membrane (5). The electrolytic membrane contains ion-exchange resin composition including fluoropolymer electrolyte polymer (I).    USE - Redox-flow secondary battery (claimed).    ADVANTAGE - The redox-flow secondary battery has excellent durability, battery efficiency, and reduced electric resistance. The electrolytic membrane has excellent permselectivity of ion.    DETAILED DESCRIPTION - A redox-flow secondary battery comprises electrolytic cell having positive electrode cell chamber containing positive electrode including carbon electrode, and positive electrode electrolyte solution having positive electrode active material, negative electrode cell chamber containing negative electrode including carbon electrode, and negative electrode electrolyte solution having negative electrode active material, electrolytic membrane as a diaphragm which carries out separation of positive electrode cell chamber and negative electrode cell chamber, and charges/discharges based on valence change of the positive electrode active material in the electrolyte solution, and the negative electrode active material. The electrolytic membrane contains ion-exchange resin composition including fluoropolymer electrolyte polymer of formula: -(CF2-CX1X2)a-(CF2-CF((-O-CF2-CF(CF2X3))b-Oc-(CFR1)d-(CFR2)e-(CF2)f-X4))g- (I).    X1-X3=halo or 1-3C perfluoroalkyl;    X4=COOZ, SO3Z, PO3Z2 or PO3 HZ;    Z=H, alkali-metal atom, alkaline-earth metal atom or amine of formula: NH4, NH3R'1, NH2R'1R'2, NHR'1R'2R'3 or NR'1R'2R'3R'4;    R'1-R'4=alkyl or arene;    R1,R2=halo, 1-10C perfluoroalkyl or fluorochloroalkyl;    a,g=0 to less than 1;    a+g=1;    b=0-8;    c=0 or 1;and    d,e,f=0-6, in which d, e, and f are not 0 simultaneously.    An INDEPENDENT CLAIM is included for electrolytic membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of redox-flow secondary battery.    Positive electrode (1)    Positive electrode cell chamber (2)    Negative electrode (3)    Negative electrode cell chamber (4)    Electrolytic membrane (5)    Electrolytic cell (6)    Redox flow secondary battery (10)