To provide a method and apparatus for water treatment that can oxidatively decompose and remove membrane contaminants efficiently using a small amount of ozone in water treatment in which water to be treated is subjected to ozone oxidation as pretreatment for RO membrane treatment to remove membrane contaminants contained in the water to be treated by oxidative decomposition, thereby preventing contamination of the RO membrane and maintaining the performance of the membrane.

This water treatment method includes an oxidation step 1 of adding ozone to water to be treated to conduct oxidation treatment, and a reverse osmosis membrane treatment step 3 of subjecting water from the oxidation step 1 to treatment through a reverse osmosis membrane. A pH adjustor is added to water to be treated so that pH of water in the oxidation step 1 or water discharged from the oxidation step 1 is 8 to 10. Since water in the oxidation step 1 or water discharged from the oxidation step 1 is adjusted to an alkaline region of pH 8 to 10 in which the reactivity of ozone and hydroxy radicals is high, membrane contaminants can be efficiently removed by decomposition with a small amount of ozone.