PURPOSE: To make ocean treatment of CO2 in the sea where water depth is shallow by clathrating the CO2 and settling the clathrated CO2 into the sea.

CONSTITUTION: An injection pipe 2 for liquid CO, is penetrated into the upper part of a clathrate forming container 1 of a vertical cylindrical shape closed in the upper part in such a manner that the liquid CO2 of the density smaller than the density of the sea water does not float and diffuse in the sea water. The clathrate forming container 1 is first arranged at about 700m water depth from the equipment on the sea. The equipment on the sea and the forming container 1 are then connected by the injection pipe 2. The CO2 accumulates in the upper part of the container 1 when is injected from an injection nozzle 3. The CO2 is stirred while the CO2 is so held as not to diffuse. The clathrate of the CO2 is formed when the CO2 solubility in the container 1 attains about 6wt.%.