PURPOSE: To simplify the structure of an above-titled power plant in which fresh water is discharged from an enclosed water tank submerged in sea water and to enable to operated the power plant at a high efficiency, by providing a chamber for reserving clean fresh water and a chamber for reserving sea water separately from each other, and connecting these chambers to said enclosed water tank via piping.

CONSTITUTION: A power plant 1 is installed at a place near the sea 3 and a river 2. Fresh water flowing through the river 2 is introduced into a fresh water reserving chamber 7 having therein a cleaning means 8 via a water inlet port 5 and a conduit 6, and clean fresh water is carried from the chamber 7 into an enclosed water tank 12 of a semipermeable membrane via a conduit 9, water turbine 10 connected to a generator 14, and a header 11. Then, it is discharged into sea water by the function of osmotic pressure. On the other hand, a sea water reserving chamber 19 having a sea-water inlet port 20 and a cleaning means 21 is formed downwardly of the fresh water reserving chamber 7, and it is connected to the water tank 12 via a pipe 22 and a resilient tube 23. Further, the chamber 7 is connected to the water tank 12 via the resilient tube 23 and a pipe 24, and a salt adding means 15 and a drainage pump 18 are connected to the water tank 12.