In this paper, a flood disaster of inundation in the Matsue City area is reported. The major mechanisms of flooding are assumed to be inundation due to heavy rainfall and topographical condition, and backwater through sewerage system due to high water level of the Oohashi River. A comprehensive numerical simulation model, which includes a simple sewerage model, is proposed in this study and applied to the Matsue City area. The inundated area and inundated water depth can be reproduced well by this model. Finally, the effects of the water level of the Oohashi River on inundated area and inundated water volume are discussed and its influence are found to be so large.