PURPOSE: To eliminate the need for feeding a soln. or concd. liquid under a high pressure by using relatively customarily used mechanical elements in a pumping apparatus of a reverse osmosis unit and effectively recovering liquid feeding energy from a concd. liquid releasing port stream.

CONSTITUTION: This apparatus has a booster pump means 55 and pump means 57 as the soln. supplying means to the reverse osmosis unit 11. These soln. supplying means are operated by liquid pressure fluid to carry out the feeding to the reverse osmosis unit 11 via this liquid pressure fluid and the recovering of the energy of the concd. liquid discharged under the high pressure from the unit 11. Plural pieces of work exchange sub-systems are built in to transmit this energy from the concd. liquid to the liquid pressure fluid and from the liquid pressure fluid to the soln. These sub-systems respectively include first and second work exchange vessels 13, 15, 17 and 19. The first work exchange vessels 13, 15 accept the soln. on their one side and the liquid pressure fluid on their other side and the second vessels 17 and 19 accept the concd. liquid on their one side and the liquid pressure fluid on their other side.