Structures of andesite slopes of columnar joints and their instability were discussed on the basis of direction and spacing of joint planes in the Katsura-jima Island area, Shimane Peninsula, Japan. Direction of joint planes strongly depends on flow structures of andesitic lava which forms several lava domes in the area. Based on shapes and dip angles of columnar blocks separated by joint planes, most of them are unstable and tend to topple due to seismic vibration or sea waves under the gravity. Considering that these characteristic structures of slopes depending on lava domes are widely distributed, they show continuous regression of steep slopes and the repetition of toppling of columnar blocks during in the Quaternary Period.