To provide a method and an apparatus for regeneration by chemical-passing of an ion exchange resin which substantially reduce the reverse regeneration rate in regeneration of the resin and are intended to improve the quality of treating water during operation using ammonia.

The method is based on chemical-passing regeneration of an ion exchange resin F from a mixed-bed type demineralizatoin column E by the use of a resin separation/mixing column D, a cationic resin regenerating column C, an anionic resin regenerating column A and a middle resin vessel B. It comprises transferring the resin F from the column E to the column D, adding a middle resin to carry out reverse washing, separating the resin F into a cationic, a middle and an anionic resin, in order from the bottom, transferring the separated anionic resin to the vessel A, carrying out further reverse washing of the residual resin to be separated, transferring the upper middle resin to the vessel B, transferring the residual cationic resin to column C to repeat the reverse washing and separation so as to transfer the mingled anionic resin to the vessel B, repeating reverse washing of the anionic resin in the column A to collect the mingled cationic resin in the bottom of the column A and carrying out chemical-passing regeneration of the purified cationic and anionic resins in the columns C and A, respectively.

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