The history of water disasters and hazards characteristics in the part of Tokyo East lowland（below sea level area）: In the present paper, the author has studied the history and characteristics of water disasters in low ground of east part of Tokyo, where the ground surface is almost below-sea-level. Tokyo East lowland was basically formed by transforming Tone River to the east and Arakawa River to the west in the Tokugawa era. Through the flood disasters from the Meiji to Showa era,Arakawa Chanel and Edogawa Chanel were newly opened and a reclamation land at Kasai was constructed to avoid flood disasters in Tokyo East lowland. However, the numerical simulations at present study reveals that the existing of fundamental risks of flood disasters in Tokyo lowland and Edogawa ward is still in danger by the lower height of dykes on the left side of Arakawa River. From a case study of numerical simulation assuming high tide with a supplemental high water by climate change around Arakawa River mouth,Edogawa ward might be easily flooded within 2 days. As there is now a high density population in Tokyo East lowland, the countermeasures against the coming flood disasters must be planned and conducted properly and urgently.