Abnormal values of an objective variable are removed. A degree of association between an objective variable and a plurality of explanatory variables is calculated. A plurality of explanatory variables having a high degree of association are extracted. A degree of independence between the explanatory variables is calculated. A plurality of candidates of explanatory variables with a high possibility of having a great effect on the objective variable are selected based on the degree of association and the degree of independence. An explanatory variable having a high rate of contribution to the objective variable is selected from among the candidates, based on the cumulative contribution rate, and a regression equation is calculated to estimate a value of the objective variable. The same processing is repeated, using the difference as a new objective variable and explanatory variables except the explanatory variable used to obtain the difference as new explanatory variables.