To provide a method which targets at a carbon dioxide having a considerable exhaust amount in an exhaust gas among increasing carbon dioxide gases and generates a useful substance by absorbing and decomposing the carbon dioxide.

An ammonium solution as a substance for absorbing the carbon dioxide is made to contact with the carbon dioxide in a misty state to absorb the carbon dioxide, and thus an ammonium carbonate aqueous solution is generated. Thereafter, an urea which is a useful substance is obtained by increasing the temperature of the ammonium carbonate aqueous solution. The carbon dioxide which is a stable substance is absorbed and decomposed to form the useful substance, and the carbon dioxide itself in an air is enabled to be decreased.