[Problem] To lessen the number of operations for cleaning a second semipermeable membrane with a chemical or the frequency of replacement of the membrane in a fresh water generation system in which biotreaetd water obtained by biotreating an organic wastewater is filtered with a first semipermeable membrane and the concentrate is mixed with seawater and then filtered with the second semipermeable membrane. [Solution] The present invention is a fresh water generation system (1) equipped with: a first semipermeable-membrane device (9) in which biotreated water obtained by biotreating an organic wastewater is filtered with a first semipermeable membrane (8) to thereby separate the biotreated water into a filtrate and a concentrate; and a second semipermeable-membrane device (13) in which the concentrate from the first semipermeable-membrane device (9) is mixed with seawater and the mixture is filtered with a second semipermeable membrane (14). The first semipermeable membrane (8) is equal to the second semipermeable membrane (14) in the tendency of the water-treating microorganism to adhere thereto, or the water-treating microorganism more readily adheres to the first semipermeable membrane (8) than to the second semipermeable membrane (14).
