A performance-lowered ion exchanger hard to recover in performance by the conventional regeneration (ion exchange resin, ion exchange membrane, or the like) is endowed with the same electric charge as the electric charge of the ion exchanger. A performance-lowered ion exchanger hard to recover in performance by the conventional regeneration owing to adsorption thereon of a charged substance is endowed with an electric charge opposite to that of the charged substance. According to the foregoing operations, the ion exchangers are recovered in performance. At least one compound selected from among organic amine compounds and organic ammonium compounds, which are capable of being endowed with an electric charge through dissociation thereof in solution, is preferably used as a rejuvenation agent for an anion exchanger.