A first aqueous solution filled in an electrolytic cell (2) is electrolyzed by applying DC voltage between the electrodes 7a and 7b in said electrolytic cell 2, to form an oxidation field short of electrons in said aqueous solution; and then, a second aqueous solution with carboxylic acid dissolved in it is mixed into the first aqueous solution in oxidation field state, so that the first aqueous solution in oxidation field state obtains electrons and is deoxidized, and the carboxylic acid is oxidized, to produce carbonic acid gas in said aqueous solution. Therefore, the present invention can be used to produce carbonic acid gas solution at a low cost easily.