a spark plug not shown and then ignited homogeneous charge combustion is performed by injecting fuel into the combustion chamber 5 from the main fuel injector 11 when the associated piston is in the intake stroke the injected fuel is mixed homogeneously with air which is drawn into the combustion chamber 5 through the associated intake ports 7a 7b and then burned the first intake port 7a of each cylinder 1a is connected with a first intake passage 15a while the second intake port 7a is connected to a second intake passage 15b the first and second intake passages 15a 15b extend through an intake manifold 15 and connect the associated cylinder 1a to a surge tank 16 the surge tank 16 is connected to a sub injector 12 fuel is injected into the surge tank 16 from the sub injector 12 when performing homogeneous charge combustion the fuel injected from the sub injector 12 consists of droplets having extremely fine diameters in comparison with the fuel injected from the main injectors 11 the surge tank 16 is connected to an air cleaner 21 through an intake duct 20 an electronically controlled throttle valve 23 which is opened and closed by a step motor 22 is arranged in the intake duct 20 an electronic control unit ecu 30 sends pulse signals to drive the step motor 22 and control.