NOVELTY - The membrane consists of an ion exchange resin in which the principal chain is crosslinked and which is derived from a metal-containing monomer, an aromatic derivative monomer, a carboxylic acid derivative monomer and a strong acid group-containing monomer.    USE - The method produces an ion exchange membrane used in solid macromolecular fuel cells, electro-chemical sensors or ion conductive elements for flat panels.    ADVANTAGE - The ion exchange membrane has low electrical resistance loss, thus reducing electrical energy loss. The membrane has a high methanol barrier property and no hydrogen fluoride is evolved.    DETAILED DESCRIPTION - An ion exchange membrane consists of an ion exchange resin of formula (1) in which the principal chain is crosslinked. The resin (1) comprises:    (1) a unit (I) derived from a metal-containing monomer;    (2) a unit (II) derived from an aromatic derivative monomer;    (3) a unit (III) derived from a carboxylic acid derivative monomer; and    (4) a unit (IV) derived from a strong acid group-containing monomer.    R1 = H, aliphatic derivative or aromatic ring derivative;    M = 1-4 valent metal;    R2 = aliphatic derivative, aromatic ring derivative or a bond;    R3-R5 = aliphatic derivative or aromatic ring derivative;    R6 = aromatic ring derivative;    R7 = aliphatic derivative, aromatic derivative or bond;    R8 = H or aliphatic derivative;    R9 = aliphatic derivative or aromatic ring derivative having acid site;    R10 = aliphatic ring or aromatic ring having three crosslinking groups;    p = 1-350;    q = 1-500;    x = 1-600;    y = 1-300;    m = 1-200; and    n = 10-10000.    An INDEPENDENT CLAIM is also included for the production of a membrane as above which comprises (all in mol ratio):    (a) mixing the monomer (I) (0.4-12) with the monomer (II) (0.2-10), the monomer (III) (0.3-13), the monomer (IV) (0.01-8.5) and a crosslinking agent (0.001-4.0); and    (b) applying a copolymerization reaction to the resulting mixture to obtain an ion exchange resin solution.