To ensure the availability of water with stably high purity for a long time by deaerating the water to be treated to remove a dissolved oxygen and at the same time, reversely osmosing the water through an osmotic membrane to desalinate it and further circulating the water through an electric regeneration desalination device after adjusting the pH value above a specific level.

The deaeration treatment is to remove a dissolved oxygen from water to be treated using a deaeration device and the reverse osmotic(RO) membrane treatment is to separate and remove salts and an ionic substance using a reverse osmotic membrane action. The treatment of the water through the deaeration and the RO treatment can be started with the deaeration treatment and the RO treatment in that order or alternatively, can be started with the RO treatment and the deaeration treatment in that order. Next the pH value of the water to be introduced into an electric regeneration desalination device is adjusted to 9 or more. The dosage of an alkali agent is dominant by setting the pH value at 9 or more. Therefore the conductivity at the inlet of the electric regeneration desalination device can be made almost constant regardless of the qualitative variation of a water supply. Besides silica in the water becomes easily ionized and consequently tends to easily move from the treated water chamber of the electric regeneration desalination device to its concentrated water chamber.