NOVELTY - A redox flow secondary battery (10) consists of positive electrode cell chamber (2) containing positive electrode (1) which contains carbon electrodes and negative electrode cell chamber (4) containing the negative electrode (3) contains carbon electrode. The battery has an electrolytic cell (6) containing electrolytic membrane (5) which used as diaphragm to separate positive electrode cell chamber and negative electrode cell chamber. The electrolytic membrane contains an ion exchange resin composition mainly having fluoropolymer electrolyte polymer.    USE - Redox flow secondary battery e.g. vanadium system redox flow secondary battery (claimed).    ADVANTAGE - The electrolytic membrane for redox flow secondary batteries has excellent ion permselectivity, low electrical resistance, excellent durability and permeation of active material ion in electrolyte solution can be suppressed and excellent current efficiency is exhibited.    DETAILED DESCRIPTION - A redox flow secondary battery has positive electrode cell chamber containing positive electrode which contains carbon electrodes, negative electrode cell chamber containing the negative electrode contains carbon electrode. The electrolytic membrane used as diaphragm to separate positive electrode cell chamber and negative electrode cell chamber. The negative electrode electrolyte solution contained in negative electrode cell chamber which contains an active material for the positive electrode electrolyte solution contained in positive electrode cell chamber which contains an active material. The discharge of the battery is based on the valence of active material in the electrolyte solution. The electrolytic membrane contains an ion exchange resin composition mainly having fluoropolymer electrolyte polymer. The redox flow secondary battery has 300-1300 g/eq mass EW of fluoropolymer electrolyte polymer.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of redox flow secondary battery.    Positive electrode (1)    Positive electrode chamber (2)    Negative electrode (3)    Negative electrode chamber (4)    Electrolytic membrane (5)    Electrolytic cell (6)    Redox flow secondary battery (10)