To provide an electrolysis method which is capable of electrolyzing even brine containing many iodine ions in the state of stable electric current efficiency.

This method for electrolyzing the brine is a chloralkaline electrolysis process of electrolyzing the brine containing alkaline earth metal ions and the iodine ions by means of an ion exchange membrane, in which the crystal containing the alkaline earth materials and the iodine is so generated that the peak of a concentration distribution comes to a depth of ≥1 μm from the surface of the perflorocarboxylic acid polymer layer of the ion exchange membrane.