An exhaust gas emitted from an engine (10) that contains nitrogen oxides is supplied to the cathode of a fuel cell (20), and a hydrogen-rich gas generated by the hydrogen gas generator (30) is supplied to the anode of the fuel cell (20). In the fuel cell (20), the nitrogen oxides are decomposed and electricity are generated by an electrochemical reaction between hydrogen and the nitrogen oxides. A hydrogen-separation membrane fuel cell, which operates at a temperature that is substantially the same temperature as that of the exhaust gas emitted from the engine (10) is used as the fuel cell (20).