NOVELTY - A cation-exchange membrane (1) comprises a fluorine-containing polymer containing a tetrafluoroethylene unit and a perfluoroethylene unit having a carboxylic acid ion-exchange group. The melt index (a) of fluorine-containing polymer is 3-50 g/10 minutes, when measured using load of 2.16 kg and orifice having internal diameter of 2.09 mm at 270 degrees C. The melt index (b) of fluorine-containing polymer is measured using orifice having internal diameter of 2.09 mm at 260 degrees C. The melt index (a) and melt index (b) of fluorine-containing polymer satisfy a specific relation (I).    USE - Cation-exchange membrane is used in electrolytic cell (claimed) used for electrolysis of water and alkali chloride used in manufacture of chlorine and alkali, generation of ozone, and fuel cell.    ADVANTAGE - The cation-exchange membrane provides electrolytic cell with stabilized current efficiency during a long-term high current density operation at 90 degrees C.    DETAILED DESCRIPTION - A cation-exchange membrane comprises a fluorine-containing polymer containing a tetrafluoroethylene unit and a perfluoroethylene unit having a carboxylic acid ion-exchange group. The fluorine-containing polymer is obtained by methyl-esterifying carboxylic acid ion-exchange groups. The melt index (a) of fluorine-containing polymer is 3-50 g/10 minutes, when measured using load of 2.16 kg and orifice having internal diameter of 2.09 mm at 270 degrees C. The melt index (b) of fluorine-containing polymer is measured using load of 2.16 kg and orifice having internal diameter of 2.09 mm at 260 degrees C. The melt index (a) and melt index (b) of fluorine-containing polymer satisfy the relation: (a-b)/(270-260) less than or equal to 0.027x a+0.19 (I). INDEPENDENT CLAIMS are included for the following:    (1) fluorine-containing polymer; and    (2) electrolytic cell (100), which consists of an anode (200), a cathode (300), and the cation-exchange membrane provided between the anode and the cathode.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of electrolytic cell.    Cation-exchange membrane (1)    Electrolytic cell (100)    Anode (200)    Cathode (300)