PURPOSE: To decrease the discharge of NOx in full through a slight exhaust gas recirculation value as well as to stabilize a driving run at time of medium- high load driving by conducting a part of exhaust gas to an injector under the specified driving conditions, and spraying this exhaust gas to a combustion chamber together with air and fuel.

CONSTITUTION: When a piston goes up and a scavenging stroke is over, an injector 48 opens and a mixture of air and fuel is sprayed into a combustion chamber 50. When the piston 16 is reached to a specified timing advance position short of a top dead center, this mixture is ignited by a spark plug 46, and the piston 16 goes down, and when an exhaust port is opened, exhaust is started. A mixture 60 feeds the injector 48 with air alone at time of low load, but if load grows larger, it mixes air and the exhaust gas together, feeding it to the injector 48. In brief, exhaust gas recirculation takes place at medium load. Therefore, a combustion temperature at this medium load is lowered, thus the discharge of NOx is reduced.