The present invention provides an ion exchange membrane can improve the current efficiency of a redox flow battery without a drop in voltage efficiency, when used in the redox flow battery. The ion exchange membrane of the present invention is an ion exchange membrane comprising a fluorinated polymer having sulfonic acid functional groups, wherein the difference (D−Dc) between the distance D between ionic clusters and the diameter Dc of ionic clusters as measured by the small angle X-ray scattering is 0.60 nm or more, and the ion exchange capacity of the fluorinated polymer is 0.95 meq/gram dry resin or more.