PURPOSE: To rapidly obtain fresh water, by providing a three-way change-over valve to the connection part of bypass piping, which connects sea water supply piping for supplying sea water to a reverse osmosis membrane module and production water piping, and the production water piping and providing a water tank on the way of the bypass piping.

CONSTITUTION: In usual operation, when the start switch of a sequencer 80 is turned ON at a time t1, a sea water supply pump P3 and a chemical liquid pump P2 are started and a sea water supply valve SV1 is turned ON at the same time. A high pressure pump P1 is started at 30sec after the time t1 and operated in a pressurized state during the operation by the outlet pressure of a reverse osmosis membrane module 12. After the elapse of a time T, a transmitted water change-over valve SV4 is turned ON to send fresh water to the drinking water change-over valve 14 of production water piping 10b as drinking water. By this method, fresh water can be supplied as drinking water within a short time.