The 2004 Niigata-ken Chuetsu earthquake caused a slope failure in Yokowatashi-area of Ojiya city, where a thin weathered tuffaceous sandstone layer was found as the slip surface. The authors investigated this slope failure by site-investigation, laboratory test of undisturbed samples and numerical analyses. The stability analysis revealed that this slope would not be ruptured by the ground water level change due to rainfall without any earthquake load, but the slope stability during the earthquake should have been influenced by the preceding heavy rainfall. In addition, it is inferred that the alternating loading by the earthquake played a significant role for the failure occurrence. [This abstract is not included in the PDF]