PURPOSE: To simply and effectively remove chlorate in brine at a low cost by allowing the circulating brine which is fed to an electrolytic cell of aq. soln. of alkali chloride in an ion-exchange membrane process to contact with a used ion-exchange membrane in the presence of hydrochloric acid.

CONSTITUTION: The aq. soln. of alkali chloride (hereafter referred to as brine) is cyclically fed to the electrolytic cell of the aq. soln. of alkali chloride in the ion-exchange membrane process and is electrolyzed. Then the chlorate in the brine is removed by allowing the circulating brine in which the chlorate is accumulated to contact with the used ion-exchange membrane in the presence of hydrochloric acid. Further it is preferable that the ion-exchange membrane is used after exchanging the ion-exchanging radical in the membrane to the H-type radical by dipping the membrane in hydrochloric acid, etc. Then the membrane composed of carbon fluoride is preferable as the membrane to be used owing to its large resistance to chlorine and the sulfonic exchange radical is preferable as the ion-exchange membrane due to its large efficiency in decomposing chlorate.