To obtain fresh water by applying constant pressure to a bag containing seawater to allow the seawater to pass through a carbon filter and a reverse osmosis membrane without using electricity at all in view of the problem of a conventional seawater desalination apparatus using a reverse osmosis membrane, in which electricity is indispensable for applying constant pressure to seawater.

A certain level of constant pressure needs to be applied to seawater to allow the seawater to pass through the carbon filter and the reverse osmosis membrane. For this purpose, the seawater is put into a pressure-resistant plastic bag, which can be easily pressurized manually by means of the force of a screw cap and a compressed coil spring, so that the seawater can pass through the reverse osmosis membrane, and thus fresh water can be obtained.