The Kanda river basin is a highly-urbanized area located next to the Tokyo metropolitan area. In this area, most ground was paved, and high density drain network has been constructed under the ground. Because of such condition, rain water concentrates rapidly to the drainage pipe, and it flows into urban river system. In this study, numerical simulation was conducted to investigate a process of urban inundation in this basin by focusing on the interaction between river and drainage system. Two rain events, Suginami torrential rain in 2005 and heavy rain event on June, 29 in 2014, were paid attention in this study. In the latter case, the rain data by X-MP radar which was provided by MLIT was applied. As a result of this analysis, it was revealed where the points with higher inundation risk lay hidden in this basin.