To solve the problem that a flooding prevention device laid in a doorway of an underground street or a subway network requires protection walls configured by loads of water stops stacked manually or mechanical and electrical mechanisms but the configuration thereof is complicated and lacked in promptness, in the case where torrential rain as heavy as Typhoon Kathleen or an earthquake as severe as the Great East Japan Earthquake occurs, in accordance with indications of people of experience or academic standing, the collapse of banks of rivers is estimated and it is said difficult to block powerful flooding water caused by the collapse, such that the way of dealing with these unexpected situations is a big task.

As measures to cope with the crisis of flooding damage, in a doorway to an underground space of a building or in a residential area, an industrial complex or a harbor where flooding damage is predicted, a protection wall body is floated from an underground accommodation case by utilizing buoyancy of water, a condition of a flooding protection system is maintained beforehand through operation of minimum man-hour, and provided as a water cut-off system of an underground installed hydraulically floating protection wall which is capable of operating even under unexpected power failure and presenting effects not only economically but also efficiently.