PURPOSE: To obtain inexpensive and good-quality potable water by supplying the warm sea water on the sea surface to a water contact device, such as cooling column for air conditioning, to bring the sea water into contact with the air and to evaporate the same, and then condensing this air via an air fin cooler by the cold waste sea water or the like used for ocean temp. difference power generation.

CONSTITUTION: The warm sea water on the sea surface is supplied to to cooling column for air conditioning or the packed column resembling thereto or other gas-water contact device 103 where the sea water is brought into contact with the air and is evaporated under the atm. pressure. This air is condensed by the cold waste sea water used for the ocean temp. difference power generation or in some cases by the particularly pumped up sea water, via the air tin cooler 20, by which the carbon dioxide, oxygen, etc., separated in an evaporator 1 are again absorbed and the inexpensive and goodquality potable water is obtd.