An internal combustion engine 10 includes a fuel injection valve 37 for injecting a gasoline fuel into a combustion chamber 25, and a spark plug 35. When the internal combustion engine is operated in a light-load region, the fuel is injected at an early and/or middle stage of an intake strode, whereby a homogeneous air/fuel mixture is formed and compressed to thereby perform a premixed-charge compression auto-ignition operation in which the fuel is auto-ignited and combusted. When the internal combustion engine is operated in a middle-load region, a spark-ignition combustion operation is performed. When the internal combustion engine is operated in a high-load region, air taken into the combustion chamber is compressed, and the fuel is injected into the compressed air, thereby performing a diffusion combustion operation in which the fuel is diffusion-combusted.