PURPOSE: To prevent the oxidative deterioration of a reverse osmosis membrane and the generation of slime and to keep a high desalting ratio by controlling the addition amount of the chlorine type oxidizing agent added to raw water so that the concn. of the residual chlorine type oxidizer at the inlet of the reverse osmosis membrane enters a specific value range as chlorine.

CONSTITUTION: The raw water filtered by a safety filter 32 is highly pressurized by a high pressure pump 12 and introduced into a reverse osmosis membrane module 14 consisting of a large number of cellulose acetate reverse osmosis elements to be separated into transmitted water and conc. water. A residual chlorine detector 24 is provided to the raw water line 22 on the upstream side of the high pressure pump 12 to detect residual chlorine in the raw water at the inlet of the reverse osmosis membrane module 14 and the motor of an oxidizer injection pump 18 is controlled on the basis of the detected value through a residual chlorine type oxidizer controller 26 to control the amount of the chlorine type oxidizer added to raw water so that the concn. of the residual chlorine in the raw water at the inlet of the reverse osmosis membrane module 14 enters the range of 0.05-0.4ppm.