A water producing system which is provided with a semi-permeable membrane treatment process A100 to produce membrane permeate A3 and concentrate A, a semi-permeable membrane treatment processB200 which is equipped with a treatment target water B branching means for branching treatment target water B2 into two or more to produce a membrane B5 and concentrate B6, and a semi-permeable membrane treatment process C300 which is equipped with a first water mixing means for mixing one of the treatment target water B with at least part of the concentrate A, and subjects the mixed water to the semi-permeable membrane treatment to produce membrane permeate C7 and concentrate C8, thereby producing fresh water from a plurality of kinds of raw water which differ in osmotic pressure.