In 2011, Typhoon No.12 caused the occurrence of many deep‐seated landslides that formed landslide dams in the Kii Peninsula. Since then, many research publications about deep‐seated landslides have been published, and methods to extract slopes that are at risk have been proposed. However, the occurrence mechanism of the deep‐seated landslides in 2011is not entirely clear. Field surveys and observation results in each area have accumulated since the disaster. In this study, these results were focused with topography, geology, and hydrology related to deep‐seated landslides, the primary causes in each area were organized, and the deep‐seated landslide occurrence mechanism was investigated from the phenomenon with many similarities in each area. The survey areas are 8 districts, Hiyamizu district, Shimizu district, Kitamata district, Akadani district, Nagatono district, Kuridaira district, Mikoshi district and Iya district, where many survey results have been accumulated in the Kii Peninsula. The phenomena with many similarities about primary causes and triggers were summarized as follows. The primary causes of these deep‐seated landslides included cliff slope resulting from gravitational slope deformation, slope failure caused by the erosion of fragile geology at the base, and loosening bedrock at deep depths because of prolonged bedrock creep. And trigger was that groundwater was supplied from a wide area even after the peak of heavy rainfall and high groundwater pressure remained for a long period. Considering these primary causes and triggers, we proposed the mechanism of the deep‐seated landslide that occurred in the Kii Mountains during Typhoon No.12 in 2011.