PURPOSE: To discard CO2 into the ocean without using a long and large-sized pipe.

CONSTITUTION: A treating ship 31 loaded with a dry ice press 13 is stopped on the ocean, and a downcomer 32 is extended from the ship 31 to a specified depth(d) where the dry ice C is hardly sublimated. A branch pipe 37 provided with a pump 38 is furnished to the downcomer 32 to form a downward seawater current (a). Gaseous CO2 discharged from the factory, etc., is liquefied and conveyed to the ship 31, the liq. CO2 is solidified by the press 13 to produce the dry ice C, and the dry ice C is entrained by the downward current (a) through the downcomer 32, forcedly and rapidly sent downward and then sent to the sea botton 36 by gravity.