To produce an ion exchange resin exhibiting remarkably reduced boron elution in particular, by highly purifying the same.

An ion exchange resin is purified by washing the same with water having been treated to contain boron at a concentration of 1 ppt or lower. The treated water is produced by treating raw water of a boron concentration of 100 ppt or lower by causing the same to flow through a reverse osmosis membrane device 1, a degassing membrane device 2, and an electric deionizer 3 one after another in that order, and subsequently causing the resulting water to flow through an oxidation device 4 employing an ultraviolet ray for oxidation, an ion exchange device 5 that is not to be reactivated, and an ultrafiltration device 6 one after another in that order. In the electric deionizer 3, a portion of the deionized water coming out of a desalination chamber is made to countercurrently flow through a concentration chamber, relative to the desalination chamber.

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