Recently, the warming on the earth advances gradually. Under such circumstances, occurrence frequency of the torrential rain increases in Tokyo. The technique of numerical prediction of the inundation is an effective tool in order to reduce damage due to such a torrential rain in urban area. In this study, the numerical prediction was conducted by using the rain data of the X-band MP radar as the first step to realize a real-time inundation forecast at the time of the localized torrential rain occurrence. Inundation in Shinjuku area that was highly-urbanized was investigated in this study by using the actual rain data that was measured locally on August 26, 2011. It was suggested that an inundation forecast could be realized soon in future.