In Kagono area, Oita Prefecture, hydrothermally altered andesitic rocks are distributed, and in the altered rocks a spa sliding is being occurred. It is necessary to detect the sliding plane in the altered rocks for the construction of barrier to prevent or stop future sliding. This spa sliding is occuring even now gently inside the altered rocks. Seven borings were made in order to find the sliding plane. The authors tried to find the sliding plane checking the boring core samples and clay minerals included in core samples. The authors found an evidence in Kagono area that there is a close relationship between slip plane and clay mineral compositions in the spa sliding. It was clear that slip plane exists near the boundary between smectite zone and mixed-mineral zone of smectite and chlorite at every boring points, and smectite content is high in both zones.
