An index calculated by weighted linear summation of normalized data requires rational sets of the weighting factors. We proposed a method to calculate the weighting factors by applying Analytic Hierarchy Process (AHP) to the results of questionnaire to experts, and applied it to a risk index called Gross National Safety for natural disasters (GNS). The questionnaire regarding the alternatives (natural disasters and disaster prevention measures) considered in GNS was addressed to three groups in charge of disaster prevention. Sets of the weighting factors determined by AHP for each group showed slightly different trend, reflecting the regional characteristics of the disasters. The sets of weighting factors for disaster prevention measures were characterized by the criteria regardless of the groups, meaning reasonable selection of the alternatives.