An operating condition value is acquired at the time of internal combustion engine startup. If the acquired condition value is one of a plurality of reference condition values for which optimum values are defined, the optimum value for the reference condition value is set as a fuel injection amount. If, on the other hand, the acquired condition value is other than the reference condition values, an interpolated value, which is interpolation-calculated by using the relationship between the reference condition values and optimum values, is set as a fuel injection amount. The angular acceleration for fuel injection according to the interpolated value, which is a physical quantity related to the operating performance of an internal combustion engine, is then determined to correct the interpolated value in accordance with the difference between the actual angular acceleration and target angular acceleration.