Flood hazard maps are quite useful for the aid to damage reduction and emergency operations for major flood crisis. In this paper, in order to derive flood hazard maps even for ungauged basins, we propose a simple method for assessing the flood inundation area based on purely topographical analyses of Digital Elevation Model (DEM), without meteorological data and hydrological models. The quality of the map derived from the proposed method is evaluated for the Fujikawa river basin against the corresponding maps given by Ministry of Land, Infrastructure, Transport and Tourism. Despite the crudeness of the model, the simulated maps are in good agreement with the reference maps. For the Chao Phraya River basin in Thailand, where an unprecedented flood disaster was caused during the 2011 monsoonal season, the simulated flood inundation map successfully reproduces the actual flooded land.