A method of absorbing sulfur oxides from flue gases in seawater, comprising adding to the seawater calcium based alkali subsequent to the absorption, and then introducing the seawater to which alkali has been supplied into a decarbonation/oxidation reactor to which an oxygen-containing gas is supplied in order to oxidize sulfur oxides in the seawater and to strip carbon dioxide from the seawater. When the seawater has been removed from the decarbonation/oxidation reactor, calcium based alkali is again added to the seawater in order to increase its pH.