An electrolytic cation exchange membrane comprises a first film made of a fluorinated polymer having cation exchange groups and a second film laminated thereon, made of a fluorinated polymer having carboxylic acid groups as its ion exchange groups and containing the following repeating units (A), (B) and (C) and having a smaller thickness and greater specific electric resistance than the first film; ##STR1## where each of X and X' is --F, --Cl or --CF.sub.3, R.sub.f is a perfluoroalkyl group having from 1 to 10 carbon atoms, M is hydrogen or an alkali metal and Y is selected from the group consisting of --CF.sub.2).sub.x, --O--CF.sub.2).sub.x, ##STR2## where each of x and y is an integer of from 0 to 10 and Z is --F or R.sub.f as defined above, provided that B/A+B+C (molar ratio) and C/A+B+C (molar ratio) are from 0.01 to 0.3 and from 0.05 to 0.5, respectively.