Abstract: The present invention provides a system for treating carbon dioxide, and a method by which such treated carbon dioxide can be stored underground at low cost and with high efficiency. The present invention' method for storing carbon dioxide underground includes: a step for pumping up to the ground groundwater from a pumping well in a deep aquifer, and then producing injection water. Carbon dioxide that has been separated and recovered from exhaust gas from a plant facility is changed into the bubbles. The bubbles are mixed with the injection water, and hereby produces a gas-liquid mixture a step for injecting into. The deep aquifer is injected into the gas-liquid mixture from the injection well. The method preferably further includes a step for dissolving a cation-forming material in the injection water, and a step for injecting the injection water, in which the cation-forming material is dissolved, into the deep acquifer at its top and above the place at which injection water has already been injected.