It is required to grasp urgent damage situation by satellite observed information. In this study, the purpose was set to extract the flood area by the 2018.07 heavy rainfall. Using the SPOT-6 observed after the disaster. The grain size index (GSI) value were applied for the extraction of the flood area. The extraction accuracy of the flood area was appropriately evaluated by using the interpretation result by the aerial photograph taken after the disaster. The extraction accuracy was improved by removing the excessively extracted area by DEM data and using spatial processing. In addition, the flood area was judged for each block by using the building polygon and the result of the extracted the flood area. It was considered that this method and improvement method was effectively used from these results to extract the flood area in the densely populated area and agricultural fields mixed.