An electrodeionization apparatus for producing deionized water comprises a deionization treatment unit including deionization chamber D and a pair of concentration chambers C1 and C2 placed adjacent to deionization chamber D on opposite sides thereof and those concentration chambers are filled with anion exchangers. The deionization chamber D is partitioned by an ion exchange membrane into first small deionization chamber D-1 adjacent to concentration chamber C1 and second small deionization chamber D-2 adjacent to concentration chamber C2. First small deionization chamber D-1 is filled with an anion exchanger. Second small deionization chamber D-2 is filled with an anion exchanger and a cation exchanger in a sequence such that the ion exchanger, through which water that is to be treated finally passes, is the anion exchanger.