This fresh water production process comprises mixing a concentrated water obtained by treating a feed water in a first semipermeable membrane unit with another feed water having a solute concentration higher than that of the concentrated water and then treating the mixed water in a second semipermeable membrane unit, wherein: both an organic matters/microbes removal line in which the concentrated water is passed through an organic matters/microbes removal unit and then mixed with the another feed water, and a by-pass line in which the concentrated water is mixed with the another feed water without being passed through the removal unit are employed; and the flow volume of the concentrated water is controlled in accordance with the concentration(s) of organic substances and/or microbes in the concentrated water on the upstream side of the removal unit and/or a change in the pressure difference between the inflow side of the first semipermeable membrane unit and the non-permeation side thereof.