An ion-exchange experiment with a countercurrent multistage fluidized bed has been carried out. The nitrate ion was effectively removed from a dilute nitric acid solution by anion-exchange resins. The outlet concentration, C-out, coincides well with the one simulated on the basis of the ion-exchange equilibrium and the effective diffusivity, D, within the resin particle. The ion-exchange equilibrium of DIAION SA10A or WA10 with a HNO3 solution in the batch experiment is expressed by the Freundlich isotherm. The value of D, measured with a fixed bed for ion exchange between an anion-exchange resin and a HNO3 solution was 1 x 10(-12) m(2)/s for SA10A and 1.5 x 10(-12) m(2)/s for WA10, respectively. Calculation of a multistage fluidized bed has shown that the removal of HNO3 is promoted with increasing stage number and increasing resin flow rate, while it decreases with increasing liquid flow rate. An increase in the stage number increases not only the removal fraction but also the ion loading of the resin.