PURPOSE: To provide a method capable of obtaining a desired material at a highest yield and operating with excellent energy efficiency in an electrodialysis applied for the production of a weak acid or a weak base.

CONSTITUTION: In the electrodialysis, the weak acid or the weak base is produced in a salt chamber by arraying successively a cation exchange membrane or anion exchange membrane and a bipolar membrane between an anode and a cathode to form a base chamber and the salt chamber or an acid chamber and the salt chamber and supplying the salt of the weak acid or salt of the weak base to the salt chamber to electrodialyze. The electrodialysis is operated batch-wisely, the electric conductivity of the salt chamber solution during the electrodialysis is measured with time and the produced weak acid or weak base is taken out at this point of time that the electric conductivity is decreased to a value based on the dissociation of the produced weak acid or weak base and is not substantially changed any more.