To provide a numerical analytic method capable of performing a further precise prediction in a method for designing an actual production process in hot working of steel by predicting the change in particle size number of the structure caused in each working step by numerical analysis.

The change of the structure accompanied by working of a material having a mixed structure or in which an area laid in a recrystallized state before working (area R) and an area laid in a non-recrystallized state before working (area N) are included is independently predicted for the area R and the area N. In a preferred embodiment, in addition to the above, the independent predictions are performed, assuming that the strain caused by working is varied between the area R and the area N and larger in the area R than in the area N by the quantity of residual strain, and the prediction is performed on the basis of both the results of these numerical analyses.