To facilitate the maintenance and control of operation and to enhance water permeation capacity by adjusting the quantity of heat of a preheater corresponding to the temp. on the outlet side of the preheater or the pressure on the inlet side of a reverse osmosis membrane apparatus.

A temp. detector is attached between a preheater 6 and a filter membrane separator 9 by the piping 7 on the outlet side of the preheater 6 to detect the temp. of raw seawater discharged from the preheater 6. A temp. controller 8 adjusts the quantity of heat supplied to the preheater 6 on the basis of the detected temp. A reverse osmosis membrane apparatus 13 is arranged and connected to the filter membrane separator 9 on the downstream side thereof through a storage tank 11 and a high pressure pump 12. Conc. water piping 15 and fresh water piping 14 are connected to the outlet of the reverse osmosis membrane apparatus 13. A product water tank 16 is connected to the fresh water piping 14 to take out fresh water as produced water. A pressure detector may be provided on the inlet side of the reverse osmosis membrane apparatus 13 in place of the temp. detector to adjust the quantity of heat to the preheater 6 on the basis of detected pressure.