To provide a condensate demineralization method and device which can form a sound mixed bed without concentration of cation resin on the resin bed bottom of a demineralizer during resin return after back washing.

In the method for demineralizing the condensate by using ion-exchange resins, the condensate is made to flow downward through the resin bed where the bottom part is filled with a part not easily separable made by selectively concentrating anion resin and cation resin of which the resin particles have terminal velocities overlapped or close to each other, and a part excluding the top part and the bottom part is filled with a mixed bed of the anion resin and the cation resin. It is desired that the part not easily separable that fills the resin bed bottom is a part of the mixed layer of resin particles which are concentrated near the separation face of the anion resin and cation resin used for condensate demineralization treatment and having a Gaussian distribution when both resins are back washed.

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