PURPOSE: To suppress the generating quantity of nitrogen oxides by returning carbon dioxide gas, generated by an oxidizing catalyst fitted to the exhaust system of an engine, to the intake side so as to decompose the carbon oxide gas into carbon and oxygen during combustion.

CONSTITUTION: Unburnt material or unoxidized material in exhaust gas is oxidized by an oxidizing catalyst 23 fitted to the exhaust pipe 19 of an engine 10. In this case, carbon dioxide gas generated by the oxidizing catalyst 23 is returned to an intake pipe 13 through a return pipeline 25. The carbon dioxide gas is then decomposed into carbon and oxygen by the heat generated during combustion. With this endothermic reaction, the combustion temperature is suppressed so as to restrain the generating quantity of nitrogen oxides.