Disclosed is a fresh water generator for fresh water production, which is effective to prevent a membrane life from becoming worse by suppressing the load on the membrane of a second semi-permeable membrane treatment facility when the concentrated water produced by the treatment of a first semi-permeable membrane treatment facility, and water to be treated with different osmotic pressure there from are mixed and treated with the second semi-permeable membrane treatment facility by the fresh water generator which comprises a means for storing the concentrated water produced from the first semi-permeable membrane treatment facility and water (B) to be treated, which is separately supplied in a storage tank A and a storage tank B in such a manner that the flow rate (mixture ratio) of the concentrated water to the water (B) to be treated is maintained constant, and supplying the concentrated water and the water (B) to be treated from each of the storage tanks to the second semi-permeable membrane treatment facility at a predetermined flow rate in at least a treatment process for treating water (A) to be treated with the first semi-permeable membrane treatment facility so as to obtain fresh water.