According to a method of desalinating sea water, sea water sampled from deep sea is injected into a decompression chamber of a reduced pressure tank having a steam chamber, steam at an atmospheric pressure superheated to a boiling temperature or higher by high-frequency heating is supplied to the steam chamber at a surrounding of the decompression chamber at an interval of a heat conducting wall therebetween, water in the sea water is evaporated by heating sea water in the decompression chamber and the evaporated water is condensed to thereby provide fresh water. Concentrated sea water remains in the decompression chamber and therefore, the method can be utilized also as a method of concentrating sea water. Salt can be obtained by further evaporating the water of the concentrated sea water as necessary.