To produce pure water of high purity by preventing the accumulation of carbonate components and M alkalinity in a system caused by concentrated water circulation while maintaining a water recovery rate when, after removing carbonates in raw water, water is passed sequentially through three RO membrane separators connected in series or through two RO membrane separators connected in series and an electric deionizer to produce the pure water.

The concentrated water of a second RO membrane separator 4 and the concentrated water of a third RO membrane separator or the electric deionizer 5 are returned to a decarbonator 1 or its upstream side. CO2 and M alkalinity can be removed in the decarbonator, and their accumulation due to the concentrated water circulation can be prevented, thereby obtaining pure water of high quality.