PURPOSE: To produce desalted water inexpensively and to reduce the size of a device, by introduing the permeated water of reduced salt obtd. by providing back pressure on the permeated water side at the fore stage of a pair of reverse osmosis modules disposed in series, directly into the raw liquid side in the rear stage.

CONSTITUTION: The sea water which is increased in pressure with a high pressure pump 1 is supplied through a sea water supply pipe into the module 2 of the 1st stage, and prescribed pressure is applied to the module 2 by means of a back pressure holding member 3. The concd. water of which the salts are concd. and which passes through the member 3 is reduced in pressure and is discharged to the outside of the system. The intermediate permeated water which is reduced in salt by permeating through the reverse osmosis membrane in the module 2 is conducted through a piping 5 to the module 6 of the 2nd stage, where the pressure necessary for operation of the module 6 is applied upon said water by means of a back pressure holding member 7, whereby the permeated water which is further reduced in salt is obtained. Said water is used as drinking water of the like through a pipe 8.