An electronic control unit 30 of a diesel engine 10 stores a relationship between a required fuel injection amount and a corresponding injection command signal. The unit 30 generates the injection command signal corresponding to the required fuel injection amount based on the stored relationship, and drives injectors 20 based on the injection command signal to inject fuel. The unit 30 includes a shifting section, which forcibly changes the engine rotational speed by temporarily shifting the fuel injection mode between a first injection mode and a second injection mode. The first injection mode is a mode in which an after injection is executed after a main injection. The after injection has less fuel injection amount than the main injection. The second injection mode is a mode in which the after injection is not executed. The unit 30 includes a correction section, which detects the variation of the engine rotational speed caused by shifting the fuel injection mode, and corrects the relationship based on comparison between the detected variation of the engine rotational speed and a previously determined standard variation of the engine rotational speed.