Inundation process in underground space which connecting to a subway station in urban area was studied numerically. Actual data sets of (1) road network, (2) drainage network and (3) underground space in the area around Shibuya Station were considered here, and a simplified numerical model was constructed. Based on the results of numerical computation, the characteristics and the weak points of the drainage system in this area were quantitatively understood. It was revealed that (1) the local area whose ground elevation is relatively lower and (2) the undergournd space are the possible area where extensive damage will result if there happenes to be the rainfall whose intensity exceeds the designed one.