Generally, degree of danger of slope falures is estimated by stability analysis. However, mainly from economical limit, when object site extends very wide area such as for the estimation of slope falure due to earthquake, estimation based on tha factors which is related to slope falure can be effective. For such a judgment, the Fuzzy Set Theory is found to be useful. Therefor, the authors introduce this theory into the estimation and apply it to the estimation of slope falures due to the Chiba-ken Toho-oki earthquake. As a result, good correlation is found betwen the Fuzzy Integral which is the index of the degree of danger and factor of safety calculated from stability analysis.