An antenna device designed to be installed on a ship, the antenna device having an elongated shape and comprising: a two-dimensional slot antenna comprising a waveguide antenna having wall surfaces and for emitting a radio wave in a direction substantially perpendicular to an emission face that is one of wall surfaces of the waveguide antenna extending in an elongated direction of the waveguide antenna, and a two-dimensional slot forming member for beam formation formed in the waveguide antenna on the emission face side, wherein the emitting direction of the two-dimensional slot antenna is orientated in a substantially horizontal direction, and the two-dimensional slot antenna is elongated in a horizontal direction perpendicular to the emitting direction; and a cylindrical radome having a substantially circular cross-section of a diameter that is substantially equal to a length of the emission face in a direction perpendicular to the elongated direction so that the waveguide antenna is contained in the radome so as to be arranged at substantially the centre of the radome; characterised in that a power feed waveguide module (30, 40) is arranged on the rear face of the waveguide antenna opposite from the emission face for supplying electric power to the waveguide antenna, the power feed waveguide module including: a first waveguide for transmitting a radio wave in a predetermined mode inputted from the outside; and a mode conversion waveguide for carrying out a mode conversion of the radio wave in the predetermined mode from the first waveguide into an emission mode for the waveguide antenna, the mode conversion waveguide including a T-shaped waveguide comprising a coupling resonator coupled to the first waveguide and a power feed resonator coupled to the waveguide antenna by power feed opening slots in the rear face of the waveguide antenna, wherein the height of the mode conversion waveguide is set to be substantially the same length as the short-side length of a main waveguide of the two-dimensional slot antenna. a two-dimensional slot antenna comprising a waveguide antenna having wall surfaces and for emitting a radio wave in a direction substantially perpendicular to an emission face that is one of wall surfaces of the waveguide antenna extending in an elongated direction of the waveguide antenna, and a two-dimensional slot forming member for beam formation formed in the waveguide antenna on the emission face side, wherein the emitting direction of the two-dimensional slot antenna is orientated in a substantially horizontal direction, and the two-dimensional slot antenna is elongated in a horizontal direction perpendicular to the emitting direction; and a cylindrical radome having a substantially circular cross-section of a diameter that is substantially equal to a length of the emission face in a direction perpendicular to the elongated direction so that the waveguide antenna is contained in the radome so as to be arranged at substantially the centre of the radome; a first waveguide for transmitting a radio wave in a predetermined mode inputted from the outside; and a mode conversion waveguide for carrying out a mode conversion of the radio wave in the predetermined mode from the first waveguide into an emission mode for the waveguide antenna, the mode conversion waveguide including a T-shaped waveguide comprising a coupling resonator coupled to the first waveguide and a power feed resonator coupled to the waveguide antenna by power feed opening slots in the rear face of the waveguide antenna,