PURPOSE: To improve the efficiency in ion exchange in upward current and regeneration in downward current by providing a partition wall body in an ion exchange tank to prevent the fall of an ion exchange resin and maintaining a fixed bed of the ion exchange resin in the upper space in the tank.

CONSTITUTION: Feed water is supplied through an inlet 17 for the liquid to be treated to raise an ion exchange resin bed, and the upper space 6 above a partition wall body 5 is filled with an ion exchange resin in a hermetically sealed tank 1 of a vertical type. The 1st communication pipe 10 in the body 5 is closed, and the 2nd communication pipe 11 is passed with water at an upward flow rate, whereby an ion exchange treatment is performed. The supply of the feed water is stopped and the pipe 10 is opened to lower the resin bed to a level A. Washing water is supplied through a washing water inlet 19 at the speed at which the top surface of the resin bed raises to a level B and the suspended matter, etc. captured in the resin bed are discharged through an outlet 20. The supply of the upward flow is stopped and after the resin settles, a regenerating liquid is passed through a regenerating liquid inlet 21 in upward current, whereby the regeneration of the resin is performed.