Fluorine-containing copolymer and an ion-exchange membrane

In such electrolysis of alkali chloride aqueous solution, it provides a fluorine-containing copolymer which can be adverse effects of impurities in the alkali chloride aqueous solution to obtain a small ion-exchange membrane. A fluorine-containing monomer having a carboxylic acid functional group, a fluorine-containing copolymer of a fluorine-containing olefins, in CClF2CF2CClFH-soluble component, common logarithm of molecular weight M (log M) is 2.0 or more components of 100% by weight, component common logarithm of the molecular weight M (log M) is 2.0 to 3.5 is 10% by mass or less, using a fluorine-containing copolymer.