An antenna device, comprising: an antenna for emitting electromagnetic waves; and a radome for holding the antenna, the radome having a double-wall structure comprising: an outer wall; and an inner wall arranged between the outer wall and the antenna and formed in a shape to substantially conform to the outer wall; wherein the inner wall is spaced from the outer wall by a gap (dc) which is substantially 1⁄4 of the emitted electromagnetic wavelength at a midpoint (Pc) on an emission face of the antenna; and the gap (dc) between the outer wall and the inner wall is substantially constant within a range from the midpoint (Pc) to prescribed positions towards both ends (Pe) of the two walls and is gradually wider approaching both ends from the prescribed positions; characterised in that: the inner wall includes a first inner wall arranged so that the gap (dc) formed up to the prescribed positions is substantially constant, and second inner walls from the prescribed positions, each having a cross-section parallel to a direction towards the centre (Po) of the outer wall from the midpoint (Pc); and the ends of the inner wall are joined to the outer wall via joint walls, wherein each joint wall is formed in a flat plate shape, and each joint wall is perpendicular to the direction connecting the midpoint (Pc) and the centre (Po) of the outer wall. an antenna for emitting electromagnetic waves; and a radome for holding the antenna, the radome having a double-wall structure comprising: an outer wall; and an inner wall arranged between the outer wall and the antenna and formed in a shape to substantially conform to the outer wall; wherein the inner wall is spaced from the outer wall by a gap (dc) which is substantially 1⁄4 of the emitted electromagnetic wavelength at a midpoint (Pc) on an emission face of the antenna; and an outer wall; and an inner wall arranged between the outer wall and the antenna and formed in a shape to substantially conform to the outer wall; wherein the inner wall is spaced from the outer wall by a gap (dc) which is substantially 1⁄4 of the emitted electromagnetic wavelength at a midpoint (Pc) on an emission face of the antenna; and the gap (dc) between the outer wall and the inner wall is substantially constant within a range from the midpoint (Pc) to prescribed positions towards both ends (Pe) of the two walls and is gradually wider approaching both ends from the prescribed positions; characterised in that: the inner wall includes a first inner wall arranged so that the gap (dc) formed up to the prescribed positions is substantially constant, and second inner walls from the prescribed positions, each having a cross-section parallel to a direction towards the centre (Po) of the outer wall from the midpoint (Pc); and the ends of the inner wall are joined to the outer wall via joint walls, wherein each joint wall is formed in a flat plate shape, and each joint wall is perpendicular to the direction connecting the midpoint (Pc) and the centre (Po) of the outer wall. the inner wall includes a first inner wall arranged so that the gap (dc) formed up to the prescribed positions is substantially constant, and second inner walls from the prescribed positions, each having a cross-section parallel to a direction towards the centre (Po) of the outer wall from the midpoint (Pc); and the ends of the inner wall are joined to the outer wall via joint walls, wherein each joint wall is formed in a flat plate shape, and each joint wall is perpendicular to the direction connecting the midpoint (Pc) and the centre (Po) of the outer wall.