A method determines a correction amount of an EGR valve opening degree. A basic correction amount map is generated indicating a correlation between rotating speed, load factor, and the correction amount, while the opening degree is maintained at a reference opening degree and the EGR rate is maintained at a target EGR rate, based on correction amounts obtained for mutually different individual combinations of the rotating speed and the load factor. Estimated values of the correction amounts not contained in the basic map are acquired by estimating the distribution of the amounts not in the basic map based on a distribution trend. An extended correction amount map is generated indicating a correlation between the estimated values of the rotating speed, the load factor, and the correction amount while the opening degree is maintained at a reference opening degree and the EGR rate is maintained at a target rate.