To provide a manufacturing method for an ion exchange membrane usable as a fuel cell separator reducing permeability against methanol and increasing catalytic activity so that a catalytic metal amount can be reduced.

By this method an ion exchange membrane usable as a separator in a fuel cell is manufactured from a film composed of at least one kind of polymer. (a) The film is activated. (b) Styrene and a chloromethyl styrene group dissolved in solvent is grafted on the activated film. (c) A grafted group in this way is sulfonated by solution containing a sulfonation agent and thereafter is functionalized through hydrolysis by basic solution so that a sulfonate part is formed in a styrene group and an alcohol part is formed in the chloromethyl styrene group.