To prevent the debris flow disasters around the active volcanos, Sabo Master Plan for disaster protection is made. In case of making the plan, it is very important to estimate the run-off sediment volume by debris flows. Especially conditions of planning such as deposit volume of pyroclastic flow, critical rainfall for the occurrence of debris flow and so on change by volcanic activity. Moreover, time series variation of conditions on sediment movement will occur in connection with volcanic activity, and will influence the occurrence of debris flow, run-off sediment volume by debris flows. I propose the necessity of basin condition investigation based on time series and showed 4 periods of volcanic activity in the case study of Mt Unzen. Especially in case that large sediment volume of pyroclastic flow are existing at greater part of basin area, run-off sediment volume by debris flow is shown by the f ollwing equation.
Q_s=(2.79×R)-112.92
where, Q_s: run-off sediment volume (×10³m³), R: accumulated rainfall (mm) .