PURPOSE: To keep the average desalting efficiency high and to effectively replace membranes when replacing used reverse osmotic membrane elements by removing the used reverse osmotic membrane element on the permeated water side and connecting and filling a new reverse osmotic membrane element on the feed water side.

CONSTITUTION: In a reverse osmotic membrane device where reverse osmotic membrane elements B-1-B-4 are arranged, connected in series from the feed water side E to the permeated water side I and they undergo oxidation deterioration by dissolved oxygen, when replacing the used reverse osmotic membrane elements B-1-B-4, the used reverse osmotic membrane element B-4 filled on the permeated water side I is removed and a new reverse osmotic membrane element B' is connected and filled on the feed water side E. Thus by only changing the replacement method of the reverse osmotic membrane elements B-1-B-4, the average desalting efficiency is kept high even when the replacement of one membrane per year is performed and the new reverse osmotic membrane element B' is prevented from being oxidized and deteriorated.