Abstract: In a water treatment method and an apparatus therefor, a high purification step comprising a precision filter and an ion-exchange resin is added to a treatment system having ozone supply units, filtering units and an ultraviolet-ray irradiating unit. Treatment objective water is treated by an ozone supply unit, a filtering unit and an ultraviolet-ray irradiating unit, so as to be brought to clean water. The clean water is treated, in super pure water, by the high purification step, and an amount of dissolved oxygen is reduced. The clean water can flow out to a location on the outside of the treatment system, from a final step, as super pure water or potable water which can immediately be used.