It is an object to provide a practical method for producing fresh water capable of continuously and efficiently obtaining a high quality fresh water that boron is efficiently removed, and provided is a method for producing fresh water, having a plurality of desalination processes in parallel where raw water or pre-treated water obtained by pre-treatment of raw water is treated by a first semi-permeable membrane unit as feed water of a first semi-permeable membrane unit, and the resulting permeate of the first semi-permeable membrane unit of which a pH is raised is treated in the second semi-permeable membrane unit as feed water of a second semi-permeable membrane unit, comprising: temporally, ,in part of desalination process A; lowering a pH of raw water or pre-treated water to be supplied to a first semi-permeable membrane unit A1 and to make a pH of the resulting permeate of the first semi-permeable membrane unit A1 lower than that of feed water of a second semi-permeable membrane unit B2 in other desalination process B to be supplied to a second semi-permeable membrane unit A2; and mixing the permeate obtained from the second semi-permeable membrane unit A2 with the permeate obtained from the second semi-permeable membrane unit B2 in said other desalination process B.