To provide a seawater desalting method which enables the efficient production of water satisfying the standards of drinking water in the concentration of boron and the like by preventing a deterioration of a second stage reverse osmosis membrane.

In the seawater desalting method for enhancing the pH of the supply water to the second stage reverse osmosis membrane device 2 to desalt seawater using reverse osmosis membrane devices 1 and 2 connected over two stages in a series direction, supply piping 9a of a washing liquid is provided on the upstream side of the reverse osmosis membrane device 2 while discharge piping 10a of the washing liquid is provided on the downstream side of the reverse osmosis membrane device 2 to enable the membrane washing of the reverse osposis membrane device 2 and, at the time of continuous operation of the devices 1 and 2 over a long period of time, membrane washing is performed at least once within the period of 90 days from the start time of continuous operation and, after the washing liquid is supplied to the reverse osmosis membrane device 2 at the time of stop of operation to remove the metal compound deposited on the surface of a reverse osmosis membrane, the reverse osmosis membrane device 2 is filled with clean water of which the pH is adjusted to preserve the reverse osmosis membrane. By this constitution, the deterioration of the reverse osmosis membrane can be prevented and desalted treated water fitted to the standards of drinking water can be produced efficiently.