PURPOSE: To efficiently remove suspended matter and to shorten the time for backwashing, by alternatively or concurrently performing the introduction of backwashing water from an intermediate water-collecting mechanism and the introduction of backwashing water from a lower water-collecting mechanism.

CONSTITUTION: In backwashing an ion-exchange column, the introduction of backwashing water from an intermediate water-collecting mechanism C2 and the introduction of backwashing water from a lower water-collecting mechanism C1 are alternatively or concurrently performed. Hence, partial backwashing and entire backwashing are repeated, and resin at an upper part is backwashed at a high speed by the introduction of water from the intermediate water-collecting mechanism C2 during said entire backwashing, so that efficient backwashing can be performed. Consequently, the time for backwashing can be shortened, and the size of the apparatus can be made small.