A device of repairing a damaged area in an underwater wall region of a container or tank, in particular in the wall region of a tank of a nuclear reactor installation. The device has a guide system that can be mounted along a side wall, at a distance therefrom, and can be secured thereto. At least one first carriage is fitted onto the track of the guide system and movable in a longitudinal direction of the guide system. On the carriage there is displaceably mounted a receptacle for a repair overlay, which can be applied with an adhesive surface to the wall region containing the damaged area. At least one suction mount that is connected to a suction line is disposed on the first carriage and can be suctioned to the side wall.