PURPOSE: To highly purify water with a possibly simplified treatment by adding sodium hypochlorite to raw drinking water, filtering the water to separate suspended particles, and then permeating the water through a reverse osmosis membrane under pressure.

CONSTITUTION: Sodium hypochlorite is added to raw drinking water and mixed. Accordingly, bacilli, bacteria, and algae are sterilized, inorg. components and org. matter are oxidized and decomposed, or the oxide deposits are formed. Then the water is filtered to remove suspended matter and permeated through a reverse osmosis membrane under pressure. Accordingly, dissolved components such as Na+, Cl-, HClO, ClO-, etc., formed by the addition of the sodium hypochlorite, similar dissolved components (excluding Na+) formed by the addition of bactericidal chlorine when city water is used as the raw water, and various dissolved components such as heavy metal ions resulting from raw water or the elution of a pipeline are completely removed, and highly purified water is obtained. Consequently, highly purified water can be continuously obtained with simplified treatment.