According to the present invention, since a pretreatment mechanism can be operated while inhibiting the pretreated water from fluctuating in pressure, a fresh-water production apparatus is obtained in which the desalting mechanism can be stably operated. Furthermore, the period of water supply or flushing in the step of washing the pretreatment mechanism can be shortened, and the load on the lines where washing is not being performed is hence reduced. Consequently, a fresh-water production apparatus is obtained in which the pretreatment mechanism can be operated while inhibiting the pretreatment membranes from being fouled.