There have occurred a world-wide anxiety about global warming due to the increasing concentration of CO₂ in atmosphere. Among various countermeasures argued is the fixation of CO₂ in deep sea. This measure seems to be promising from the viewpoint of the huge amount of CO₂ to be treated, provided that the influence on the deep-sea environment be acceptable and that the cost and/or energy consumption would not make an impact on the economic situation. In this paper, CO₂ transportation system is studied at first, which consists of CO₂ loading on board at port, transport to planned sea area and sending CO₂ into deep sea. Three types of system are planned conceptually, each of which corresponds i) solid, ii) liquid and iii) hydrate for the CO₂ state of cargo, respectively. As a result, the fatal technical difficulties are not found so far as the transportation system. Furthermore, considering the merits and demerits of the three types comparatively, the one for liquid CO, is estimated main to be carried on to the next phase of study for the present. Secondly, the experimental investigation is performed. A pressure vessel filled with highly pressed artificial sea water is situated in the room of steady temperature. And liquid CO₂, is slowly injected. Consequently, it is experimentally confirmed that liquid CO₂ become heavier than sea water under condition of the waterdepth of more than 3000 meters. This is an evidence of the self-sinking of liquid CO₂ in deep sea more than 3000 meters.