To obtain treated water of high quality by efficiently reducing the concentration of TOC in water simultaneously with performing long-term stable treatment by suppressing bio-fouling caused in a recovery system and preventing the lowering of the flux of an RO membrane by reducing organic matter using an RO membrane separator after the hardness component of organic matter-containing wastewater containing a large amount of the hardness component discharged from an electronic device manufacturing factory and other various fields is reduced without using an ion exchange resin column.

After a sterilant is added to organic matter-containing water, the organic matter-containing water is passed through a first RO membrane separator 2 having a hardness component removing function and oxidation resistance and, after alkali is added to first RO treated water to adjust the pH thereof to 9.5 or above, tile left sterilant is removed by an activated carbon column 4 and the first RO treated water is subsequently passed through a second RO membrane separator 5 to be treated.