Abstract: The present invention relates to a cation exchange membrane comprising a fluorine-containing polymer containing a tetrafluoroethylene unit and a perfluoroethylene unit having a carboxylic acid-type ion exchange group, wherein a melt index a of the fluorine-containing polymer from methyl esterifying the carboxylic acid-type ion exchange group measured under the condition at 270° C., a load of 2.16 kg and an orifice inner diameter of 2.09 mm is 3 to 50 g/10 min, and the melt index a and a melt index b of the fluorine-containing polymer from methyl esterifying the carboxylic acid-type ion exchange group measured under the condition at 260° C., a load of 2.16 kg and an orifice inner diameter of 2.09 mm satisfy the relation represented by the following expression (1): (a?b)/(270?260)?0.027×a+0.19??(1).