Predicting the sediment volume of debris flows is the most important factor for designing debris flow control structures and identifying debris flow prone areas. In general, it has been considered that the volume of a debris flow should be controlled by both the sediment transport capacity and removable sediment volume of river bed. Sediment transport capacity should be determined by sediment concentration of debris flow and water volume contributing to debris flow. According to theoretical studies, the “equilibrium sediment concentration of debris flow” has been proposed and verified by the flume experiment. However, Water volume contributing to debris flow has not been fully examined based on field data. Recently, topographic data having a high spatial resolution and rainfall data having a high temporal resolution has been stored. In this study, we estimated sediment concentration of debris flow using LiDAR data and radar rainfall data and then compared with the equilibrium sediment concentration calculated by using the debris flow theory to examined water volume contributing to debris flow. As a result, it is considered that rainfall amount of less than 1-hour may contribute to sediment volume of debris flow.