In a NOx removal catalyst used for removing nitrogen oxides in flue gas, when a silica component as an inhibitor that causes an increase in a SO2 oxidation rate accumulates on a surface of the catalyst, the silica component accumulating on the surface of the catalyst is dissolved, thereby regenerating the catalyst. The system generates an eliminating entry for the intercompany transaction at the time and every time it receives an approval, thereby providing timely and accurate financial information for the holding company. The second propulsion mechanism is arranged to generate thrust by an electric motor.