An ion exchange fluorocarbon resin membrane having a storage modulus (JIS K-7244), when substituted with tributyl ammonium ions, at a temperature of not less than Tc represented by the following general equations (1-1) to (1-3) of 0.8.times.10.sup.6 Pa or more, and a heat of crystal fusion at 270 to 350.degree. C. of 1 J/g or less: Tc (.degree. C.)=55+Tg (EW<750) (1-1) Tc (.degree. C.)=0.444.times.EW-278+Tg (750.ltoreq.EW<930) (1-2) Tc (.degree. C.)=135+Tg (EW.gtoreq.930) (1-3) where Tg designates the peak temperature of loss tangent in the dynamic viscoelasticity measure-ment of the ion exchange fluorocarbon resin membrane (having SO.sub.3H as the ends of the side chains).