In this study, inundation process in lowland areas of Tokyo which is bordered by the Ara River and the Sumida River was investigated numerically. Numerical technique developed by the first author was applied in order to estimate how hazardous the urban lowland area under two different conditions. The state of inundation which is caused by a localized torrential rain was predicted. Inundation rocess in this area was also investigated numerically under the assumption that the overflow from the Ara River into this area occurred. As a result of these simulations, the time series of information about the flow path of inundated water were obtained, and the aracteristics of inundation in this area were made clear.