An actual maximum fuel injection rate is computed based on a falling waveform and a rising waveform of the fuel pressure. The falling waveform represents the fuel pressure detected by a fuel sensor during a period in which the fuel pressure increases due to a fuel injection rate decrease. The rising waveform represents the fuel pressure detected by the fuel sensor during a period in which the fuel pressure decreases due to a fuel injection rate increase. The falling waveform and the rising waveform are modeled by modeling functions. A reference pressure is computed based on pressure during a specified time period before the falling waveform is generated. An intersection pressure is computed, at which the straight lines expressed by the modeling functions intersect to each other. The maximum fuel injection rate is computed based on a fuel pressure drop from the reference pressure to the intersection pressure.