A fluoropolymer ion−exchange membrane which, after replacement with tributylammonium ions, has a storage elastic modulus (JIS K−7244) as measured at temperatures T<sb>c</sb> represented by the following general formulae (1−1) to (1−3) of 0.8x10<sp>6</sp> Pa or higher and a heat of crystal fusion as measured at 270 to 350°C of 1 J/g or less: T<sb>c</sb> (°C) = 55 + T<sb>g</sb> (EW<750)(1−1) T<sb>c</sb> (°C) = 0.444xEW − 278 + T<sb>g</sb> (750≤EW<930)(1−2) T<sb>c</sb> (°C) = 135 + T<sb>g</sb> (EW≥930)(1−3) wherein T<sb>g</sb> indicates a peak temperature for loss tangent in a dynamic viscoelasticity examination of the fluoropolymer ion−exchange membrane (side chains are terminated by SO<sb>3</sb>H).
