The Akatani landslide triggered by heavy rainfall during Typhoon Talas on 4 September 2011 is one of 72 deep-seated catastrophic rock avalanches in Kii Peninsula, Japan. The landslide is about 900 m in length, 350 m in average width and 66.5 m of maximum depth of the sliding surface. A rapid movement of the landslide was downward the opposite valley and formed a natural reservoir that has a height of about 80 m and a volume of 10.2 million m3. This paper presents preliminary results of the simulation of the formation process of the Akatani landslide dam by using ring shear apparatus incorporated with a computer simulation model LS-Rapid. Ring shear tests on sandstone-rich materials and mudstone-rich materials taken near the sliding surface indicated that a rapid landslide was triggered due to excess pore water pressure generation under shear displacement control tests and pore water pressure control tests. The pore water pressure ratio (ru) due to rainfall was monitored from 0.33 to 0.37 in the ring shear tests on rainfall-induced landslides, approximately. Particularly, the formation process of the Akatani landslide dam and its rapid movement were well simulated by the computer model with physical soil parameters obtained from ring shear experiments. The actual ratio of pore water pressure triggering landslides was 0.35 in the computer simulation model. The results of the Akatani landslide simulation would be helpful to the understanding of failure process of deep-seated landslide induced by rainfall for future disaster mitigation and preparation in the area.