Daiiti‐Kasima Seamount is a flat‐topped and odd shaped seamount of 3,000–4,000 meters in relative height, rising in the axis of the Japan Trench, situated 220 kilometers east of Tokyo. Survey of this seamount had been conducted many times since 1934. Mogi and Nishizawa (1980) had provided a hypothesis that breakdown had taken place, and the seamount was divided into two parts by a large NNE‐SSW fault, while the western half of the seamount had subsided into the Japan Trench.