For the effective use of an ion exchange resin, a counter-current multistage fluidized bed has been developed. An appropriate design of the downspout, which works to let particles flow down to next stage, is essential to obtain a stable resin flow. From an experimental study on the particle holdup and pressure drop in the multistage fluidized bed, a method to determine the downspout size has been developed. The relation between the liquid velocity within the downspout and the superficial liquid velocity has been derived from considering the pressure balance between the part of the fluidized bed and that of the downspout.