A series of combustion forms including initial low-temperature combustion, premixed combustion, and diffusive combustion are performed when an engine operates under a low load and a medium load. The initial low-temperature combustion is carried out by performing a small-amount injection while performing an operation for lowering the encounter rate between oxygen and a fuel spray in a cylinder, and thereby the heat generation rate is kept low and the amount of NOx generated is suppressed. The premixed combustion is carried out as fuel receives heat in the initial low-temperature combustion, and the amount of smoke generated is suppressed. The diffusive combustion is accomplished as fuel travels through the combustion field of the premixed combustion, and by controlling the fuel injection timing thereof, it is possible to suitably control the timing at which the heat generation rate reaches its maximum in the aforementioned series of combustion. Thereby, it is possible to concurrently suppress the amount of NOx generated, suppress the amount of smoke generated, and secure the required torque.