NOVELTY - A redox flow secondary battery (10) has an electrolytic cell (6) which consists of a positive electrode cell chamber (2) containing a positive electrode (1), a negative electrode cell chamber (3) containing a negative electrode (4), and an electrolytic membrane (5) having a laminated structure of three or more layers containing an ion-exchange resin composition which contains a fluoropolymer electrolyte polymer having a structural unit (I), as main component, in which the equilibrium moisture content of the outer layer adjacent to the positive and negative electrodes is higher.    USE - Redox flow secondary battery e.g. vanadium-type redox flow secondary battery (claimed).    ADVANTAGE - The electrolytic membrane has excellent ion permselectivity, high proton (hydrogen ion) permeability and oxidation-resistant deterioration property (hydroxy-radical tolerance), low electrical resistance, and suppresses permeation of the active material ion in electrolyte solution. The redox flow secondary battery has high current efficiency.    DETAILED DESCRIPTION - A redox flow secondary battery has an electrolytic cell which consists of a positive electrode cell chamber containing a positive electrode consisting of carbon electrodes, a negative electrode cell chamber containing a negative electrode consisting of a carbon electrode, and an electrolytic membrane as a diaphragm which carries out the isolation separation of the positive electrode cell chamber and the negative electrode cell chamber. The positive electrode cell chamber contains positive electrode electrolyte solution containing an active material, and the negative electrode cell chamber contains negative electrode electrolyte solution containing an active material. The electrolytic membrane has a laminated structure of three or more layers containing an ion-exchange resin composition which contains a fluoropolymer electrolyte polymer having a fluoro group-containing structural unit of formula: -(CF2CX1X2)a-(CF2-CF((-O-CF2-CF(CF2X3))b-Oc-(CFR1)d-(CFR2)e-(CF2)f-X4))g- (I), as main component, in which the equilibrium moisture content of the outer layer adjacent to the positive electrode and the negative electrode is higher than the equilibrium moisture content of the intermediate layer which does not adjoin the positive electrode and the negative electrode.    X1-X3=halo or 1-3C perfluoroalkyl;    X4=COOZ, SO3Z, PO3Z2 or PO3HZ;    Z=H, alkali-metal atom, alkaline-earth metal atom, or amines (NH4, NH3R1, NH2R1R2, NHR1R2R3, NR1R2R3R4);    R1-R4=alkyl or arene;    a=0-1 (excluding 1);    g=0-1 (excluding 0);    a+g=1;    b=0-8;    c=0 or 1;and    d,e,f=0-6.    (i) when X4 is PO3Z2, Z is same or different, and R1 and R2 are halo, 1-10C perfluoroalkyl or fluorochloro alkyl; and (ii) d, e, and f are not 0 simultaneously. An INDEPENDENT CLAIM is included for electrolytic membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows a 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)