PURPOSE: To obtain a fluorine-contg. ion exchange resin membrane suitable for use as a diaphragm for electrolysis and having improved mechanical strength, etc., by molding a fluorine-contg. ion exchange resin having a specified high- molecular side chain introduced thereinto, into a membrane.

CONSTITUTION: In the production of a fluorine-contg. ion exchange resin having ion exchanging functional groups, e.g., by copolymerizing a vinyl monomer such as tetra-fluoroethylene and a fluorovinyl monomer having an ion exchange group such as sulfonic or carboxyl group, a microvinyl monomer having an MW of 300W10,000 and capable of giving 10C or higher side chain is copolymerized as a structural unit. The resulting ion exchange resin is molded into a membrane to obtain the desired fluorine-contg. ion exchange resin membrane. Examples of the microvinyl monomers are those of formulasI, II and III, wherein m, n, p are each 5 or more.