A computer simulates a relationship between a fuel-rate of fuel which collides with the cavity of the piston and the fuel injection timing, and/or the relationship between the fuel-rate of fuel which collide with the cylinder inner wall and the fuel injection timing. A range of the fuel injection timing in which the combustion state is stable is computed based on the simulation results. This range of fuel injection timing may be defined as the measuring range in which the measure points are arranged. Furthermore, the fuel injection timing is varied gradually around an upper and lower limit values of the measuring range and the engine torque is measured. The measuring range is corrected so that the combustion state becomes stable.