To alleviate or eliminate a problem of unburned fuel discharged to the outside of a cylinder in a case in which an air fuel mixture is insufficiently combusted in a combustion chamber. During the expansion stroke in which a high voltage is applied to an ignition plug via an ignition coil, and a spark discharge is caused to occur at the ignition plug, thereby the air fuel mixture in the combustion chamber is ignited and combusted, in a case in which deterioration of combustion state is detected, a microwave electric field is created in the combustion chamber prior to an opening timing of an exhaust valve at an end stage of the expansion stroke, thereby plasma is generated and enlarged in the combustion chamber.