An ion exchange membrane comprising: 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 the layer A comprises a polymer having a sulfonic acid group derived by hydrolysis from a compound represented by the following formula ; and the layer B comprises a polymer having a carboxylic acid group derived by hydrolysis from a compound represented by the following formula : CF 2 =CF-(OCF 2 CYF) a -O-(CF 2 ) b -SO 2 F wherein a represents an integer of 0 to 2, b represents an integer of 1 to 4, and Y represents -F or -CF 3 ; and CF 2 =CF-(OCF 2 CYF) c -O-(CF 2 ) d -COOR wherein c represents an integer of 0 to 2, d represents an integer of 1 to 4, Y represents -F or -CF 3 , and R represents -CH 3 , -C 2 H 5 , or -C 3 H 7 , characterized in that the 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)], the ion cluster diameter being determined as described in the description, 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/m 2 . 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 the layer A comprises a polymer having a sulfonic acid group derived by hydrolysis from a compound represented by the following formula ; and the layer B comprises a polymer having a carboxylic acid group derived by hydrolysis from a compound represented by the following formula : CF 2 =CF-(OCF 2 CYF) a -O-(CF 2 ) b -SO 2 F wherein a represents an integer of 0 to 2, b represents an integer of 1 to 4, and Y represents -F or -CF 3 ; and CF 2 =CF-(OCF 2 CYF) c -O-(CF 2 ) d -COOR wherein c represents an integer of 0 to 2, d represents an integer of 1 to 4, Y represents -F or -CF 3 , and R represents -CH 3 , -C 2 H 5 , or -C 3 H 7 , characterized in that the 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)], the ion cluster diameter being determined as described in the description, 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/m 2 . <EOS>