This paper presents a definition of estimative index for the possibility regression analysis. The estimative index quantitatively indicates the ability to explain of model and is indispensable for objectively estimating the model. An optimum model could be selected if there is an objective estimating index such as this estimative index when, for example, several different modeling for the same object have been attempted. Since such is an important matter of concern when it is tried to apply the possibility regression analysis to a practical problem, existence of the estimating index for a model such as estimative index is considerably significant. The estimative index defined here is aimed at indicating the ability to explain of possibility regression model upon noticing the fact that the possibility region for variable is estimated by possibility regression analysis.