In recent years, the frequency of floods exceeding the planned magnitude has been increasing in Japan which is forcing society to cope with such increasing external forces. In this study, in order to assess available storage functions in sub-basin scales for class A river systems, existing storage capacity was evaluated by flood control storage of dams as well as hydropower and agricultural water dams, additionally by retarding basins and agricultural ponds, and paddy field storages. As a result, the storage capacity by basin flood control measures was quantified in terms of equivalent rainfall evaluation (mm) storage volume (m³) by catchment area (km²), which is expanded to a nationwide map to make possible to evaluate whole river basins in Japan on the same scale. These outputs can be utilized to enhance river basin flood management such as to select possible new dam sites and so on.