In the Kii Mountains, deep‐seated landslides have repeatedly occurred in the past. To develop a policy for the prevention of deep‐seated landslides in this area, the volume and frequency of occurrence of landslides need to be measured. Although the age of landslides and debris flows have been actively studied, such as by using radio‐carbon dating, tephrochronology, and dendrochronology, only a few studies have focused on the age of the deep‐seated landslides. This study aims to estimate the age and frequency of deep‐seated landslides in the Kii Mountains. Through geographical interpretation using aerial laser survey data, 19 slopes were selected for obtaining the samples by extracting the sedimentary terraces adjacent to the site of the deep‐seated landslide. Based on the radiocarbon dating results of 42 samples, the relationship between the clarity of shape of the sliding cliff and radiocarbon dating was considered, and the frequency of deep‐seated landslides in the Kii Mountains was examined.