This paper proposes a method to evaluate the flood risk of each district in a municipality to assist disaster management personnel. The method is specifically for municipalities in a mountainous region where insufficient information is available for practical disaster management. Using this method, we conducted inundation analysis for multiple patterns of rainfall and discharge using a rainfall-runoff-inundation model, and estimated the maximum inundation depth and duration. Based on the estimation, we developed a “flood diagnostic chart” to evaluate district-level flood risk, additionally considering other indicators. Moreover, we located flood hotspots, which are areas requiring extra precautions because of the high flood risk for districts.