In a water treatment system with a function of filtering raw water, a filter can be securely and efficiently sterilized without using dangerous chemicals such as hydrochloric acid and sulfuric acid. The water treatment system 1 has a strongly acidic aqueous solution generating unit 17 provided with an electrolytic bath for storing a solution containing an electrolyte, the electrolytic bath having a cathode chamber and an anode chamber which are partitioned by an ion-permeable membrane, a cathode and an anode disposed in the cathode chamber and the anode chamber, respectively, and a voltage applying section for applying a direct current voltage between the cathode and the anode; a strongly acidic aqueous solution feeding passage 18 for connecting the strongly acidic aqueous solution generating unit 17 to a raw water inlet of a reverse osmosis membrane module 2 housing a reverse osmosis membrane to supply a strongly acidic aqueous solution generated in the strongly acidic aqueous solution generating unit 17 into the reverse osmosis membrane module 2; and potentiometers V1 and V5 for measuring a oxidation-reduction potential of the strongly acidic aqueous solution which has passed through the reverse osmosis membrane module 2.