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 containing positive electrode active material, negative electrode cell chamber (4) containing negative electrode (3) including carbon electrode and negative electrode electrolyte solution containing negative electrode active material and electrolytic membrane (5) containing ion-exchange resin composition including perfluorocarbon sulfonic acid-type resin having structure (I).    USE - Redox-flow secondary battery (claimed).    ADVANTAGE - The redox-flow secondary battery has excellent ion-selective permeability and current efficiency, and low electrical resistivity. The electrolyte membrane in the redox-flow secondary battery suppressed ion permeability of active substance without detracting from proton permeability.    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 containing positive electrode active material, negative electrode cell chamber containing negative electrode including carbon electrode and negative electrode electrolyte solution containing negative electrode active material, and electrolytic membrane as a diaphragm which carries out separation of positive electrode cell chamber and negative electrode cell chamber. The battery 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 has ionic conductivity of 0.05 S/cm or more, measured at 100 degrees C under relative humidity of 50%. The equivalent mass of the perfluorocarbon sulfonic acid-type resin is 250-1500 g/eq. The electrolytic membrane contains ion-exchange resin composition including perfluorocarbon sulfonic acid-type resin having structure of formula: -(CF2-CF2)a-(CF2-CF(-O-(CF2)m-SO3H))g- (I). The equivalent mass of the perfluorocarbon sulfonic acid-type resin is 250-1500 g/eq.    a,g=more than 0 to 1;    a+g=1;and    m=1-6.    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)