To prevent the scale deposition of hard components in a concentration chamber and to keep deionization performance by installing a softening treatment column in a concentrated circulating water circulating line circulated in the concentration chamber.

An electric deionized water producing apparatus (FDI apparatus) 1 is composed of deionization chambers filled with ion exchangers such as ion exchange resins and ion exchange fibers and concentration chambers separated from deionization chambers through ion exchange membranes, which are installed alternately. All concentrated water discharged from the concentration chambers and part of raw water are introduced into a concentrated water storage tank 2 and stored. The concentrated water in the tank 2 is supplied to a softening treatment tank 3 by a circulating pump, softened water obtained by removing hard components in a softening treatment column 3 is supplied to the concentration chambers and electrode chambers of an FDI apparatus 1. A trace quantity of hard components existing in the concentrated water are removed in the column 3, circulated, and used and can prevent the generation of scale in the concentration chambers even after being used for a long time.