PURPOSE: To maintain sterilization and performance of a reverse osmosis equipment and to inhibit oxidative deterioration due to chlorine of a reverse osmosis membrane by reducing chlorine having oxidative action with sulfite intermittently injected before feeding chlorine to the reverse osmosis membrane.

CONSTITUTION: Chlorine is continuously injected to raw water and also in the rear flow thereof, sulfite not less than equivalent mol. of chlorine incorporated in raw water is intermittently injected before a reverse osmosis equipment 8. Next injected of sulfite is stopped until chlorine is detected in the condensed water that flows out from the reverse osmosis equipment 8 when sulfite is not injected. Thereby sterilization is effectively performed and also oxidative deterioration due to chlorine of a reverse osmosis membrane is prevented and desalting performance can be maintained in a range over a long period.