A change amount per unit time of an engine output command for controlling engine output of an internal combustion engine is calculated as an engine output increasing rate, and a power supply device is controlled so that power corresponding to the calculated engine output increasing rate is supplied to a combustion promoter generation device. The combustion promoter generation device generates a combustion promoter through the power supplied from the power supply device to supply the combustion promoter to a combustion chamber of the internal combustion engine, and a generation amount of the combustion promoter increases as the supplied power increases. In this manner, the generation amount of the combustion promoter is adjusted.