2 正会員 博(工) 関西大学助教 環境都市工学部 都市システム工学科(〒564-8680 吹田市山手町3-3-35) 3 正会員 博(工) 関西大学教授 環境都市工学部 都市システム工学科(〒564-8680 吹田市山手町3-3-35) 4 正会員 Ph.D. 京都大学大学院教授 工学研究科社会基盤工学専攻(〒615-8540 京都市西京区京都大学桂) Mega cities in Japan, for instance Tokyo, Nagoya or Osaka, are susceptible to floods, because they locate in lowlands which are below the flood water level of rivers. These areas are prone to floods caused by rainwater, river-water and seawater. Some cities are fully urbanized, and underground shopping arcades and subway stations are there. These underground spaces are also flood-prone areas. In this paper, Tsunami flooding in underground space caused by the Nankai Trough Great Earthquake is discussed by using calculations of surface and underground inundation. Results show that entrance location of flooded water intrusion by Tsunami flooding is quite different from a pluvial flooding case and that the amount of inflow is very huge. This means that counter measures for underground inundation should be planned for each kind of flood disaster.