[Object] Conventionally, measures for securing stability of a ship when it is damaged are required.[Solution] The invention is characterized by having a remotely openable seawater inlet means provided to a lowermost watertight deck that forms a void space at the bottom of the ship. Thereby, when a side shell plate or the like of the ship is damaged and seawater enters the ship, the seawater that has entered the ship can be introduced into the void space by opening the seawater inlet means provided to the lowermost watertight deck, so that the void space, which usually provides a huge auxiliary buoyancy, can be made to function as a sort of a seawater ballast tank, whereby the ship's stability can be recovered.