To expand a lean burn limit in a lean burn range, and promote atomization of sprayed fuel and prevent deposit adhesion of fuel on walls outside of the lean burn range.

When an operating mode is in a lean burn range, an assist air control valve 14 inserted in an assist air passage 15 is fully closed to sop an assist air feed to an air assist injector 5, so that turbulence of swirl by assist air is prevented and an ignition quality is improved by stratification of an air-fuel mixture. In contrast, when the operating mode is outside of the lean burn range and a target air-fuel ration tends to be on the lean side, the assist air control valve 14 s throttled to regulate the assist air feed and improve the ignition quality by stratification of the air-fuel mixture. In accordance with transition of the target air-fuel ration from a stoichiometric side to a rich side, the opening angle of the assist air control valve 14 is increased so that increased quantity of assist air is fed, atomization of fuel by the assist air is promoted, and depositing of adhesion of fuel on walls is prevented.