Disclosed is a water producing system which produces fresh water by treating water to be treated (A) in a first semipermeable membrane treatment facility (2) and produces fresh water by mixing concentrated water generated in the treatment in the first semipermeable membrane treatment facility (2) into water to be treated (B) and treats mixed water thereof in a second semipermeable membrane treatment facility (3), the water producing system being characterized in that a bypass line for mixing the water to be treated (A) into the water to be treated (B) or the concentrated water without going through the first semipermeable membrane treatment facility (2) is provided so that the second semipermeable membrane treatment facility (3) can run even when the treatment cannot be performed, for example, due to the occurrence of a problem in the first semipermeable membrane treatment facility (2).
