The ion exchange membrane according to the present invention comprises a layer A comprising a fluorine-containing polymer having a sulfonic acid group and a layer B comprising a fluorine-containing polymer having a carboxylic acid group, wherein a ratio of an ion cluster diameter of the layer B after electrolysis under the following electrolysis conditions to the ion cluster diameter of the layer B before the electrolysis [(the ion cluster diameter of the layer B after the electrolysis)/(the ion cluster diameter of the layer B before the electrolysis)] is 0.83 to 0.95:(Electrolysis Conditions)in a zero-gap base electrolytic cell where the ion exchange membrane is disposed between an anode chamber to which a 3.5 N aqueous sodium chloride solution is supplied and a cathode chamber to which a 10.8 N aqueous sodium hydroxide solution is supplied, electrolysis is performed for 7 days under conditions having a temperature of 85° C. and a current density of 6 kA/m2.