Induced by a shock of earthquake occurred on December 17, 1987, many slope failures happened on hillside slopes of the Kazusa-Hills, Chiba Prefecture, which consist mainly of thick mudstone and their alternations of Plio-Pleistocene. By the type of movement, the influence of joint and stratification, the slope failures can be classified into 5 groups; namely, rock fall, slide on joint surface formed in mudstone, collapse in semi-consolidated sandstone, toppling and bedrock slide. Strength parameters were determined by direct shear tests on three kinds of rock samples concerned in failures: that is, jointed mudstone, semi-consolidated massive sandstone and well-laminated sandstone. The stability analysis was made by using limit equilibrium method on 12 slope failures and one bedrock slide. The original factor of safety before the earthquake shock is inferred to be 1.0 to 2.0, and its relationship with the critical acceleration shows the necessity and usefulness of slope stability maps.