A fuel injection system has a common rail storing fuel, a fuel injector injecting the fuel, a fuel-passage supplying the fuel to the fuel injector, and a fuel-pressure sensor detecting a fuel pressure. An ECU acquires a waveform of a fuel pressure representing a change of a fuel pressure based on the detected fuel pressure at the fuel injection. The ECU calculates a speed of a fuel pressure wave forming the waveform of the fuel pressure based on a period of a pulsation of the waveform of the fuel pressure and a fuel passage length, and a fuel density based on the speed of the fuel pressure wave. The ECU further calculates a fuel cetane number, and a kinematic viscosity of the fuel based on the fuel density and the fuel cetane number. The ECU judges fuel properties based on the kinematic viscosity of the fuel.