The mast device for a submarine comprises a guiding means, a vertically traversable mast guided therein and information means which are provided at the mast upper end and which are in electrical interaction with the inside of the submarine for emitting and receiving information. The information means are arranged in a buoyant unit held on the mast upper end without connection to this. The buoyant unit is fastened to a flexible coupling means which may be let out and retracted and which is of the length essentially of the maximum desired observation depth of the submarine.