An antenna device (1300, 1400) comprising a shield box including a dielectric (300, 400) in a prismatic shape and a conductor (302a, 302b, 402a, 402b) coating a portion of the dielectric, wherein, assuming one surface of the shield box as a main surface and a surface opposing the main surface as a back surface, the main surface has a structure in which patterns formed by a planar shape of the conductor (302a, 302b, 402a, 402b) coating the dielectric and a planar shape of the dielectric not coated with the conductor coincide with each other when rotated around a geometric center of gravity of the main surface by 180 degrees, wherein a fraction of the dielectric coated with the conductor (302a, 302b, 402a, 402b) is larger on the back surface than on the main surface, when defining an x-axis and a y-axis parallel to the main surface and orthogonal to each other and also defining a z-axis perpendicular to the main surface, dimensions of the shield box are equal to or less than half a wavelength of a frequency used in directions of the x-, y-, and z-axes, and the main surface has a structure in which the planar shape of the conductor (302a, 302b, 402a, 402b) coating the dielectric includes two trapezoids or triangles not axially symmetric about a first centerline equally dividing the main surface in two without overlapping the conductor (302a, 302b, 402a, 402b), lower bases of the two trapezoids or triangles adjoin mutually opposing sides of the main surface, and the planar shape of the dielectric (300, 400) not coated by the conductor (302a, 302b, 402a, 402b) is not axially symmetric about a second centerline equally dividing the main surface in two and orthogonal to the first centerline. wherein, assuming one surface of the shield box as a main surface and a surface opposing the main surface as a back surface, the main surface has a structure in which patterns formed by a planar shape of the conductor (302a, 302b, 402a, 402b) coating the dielectric and a planar shape of the dielectric not coated with the conductor coincide with each other when rotated around a geometric center of gravity of the main surface by 180 degrees, wherein a fraction of the dielectric coated with the conductor (302a, 302b, 402a, 402b) is larger on the back surface than on the main surface, when defining an x-axis and a y-axis parallel to the main surface and orthogonal to each other and also defining a z-axis perpendicular to the main surface, dimensions of the shield box are equal to or less than half a wavelength of a frequency used in directions of the x-, y-, and z-axes, and the main surface has a structure in which the planar shape of the conductor (302a, 302b, 402a, 402b) coating the dielectric includes two trapezoids or triangles not axially symmetric about a first centerline equally dividing the main surface in two without overlapping the conductor (302a, 302b, 402a, 402b), lower bases of the two trapezoids or triangles adjoin mutually opposing sides of the main surface, and the planar shape of the dielectric (300, 400) not coated by the conductor (302a, 302b, 402a, 402b) is not axially symmetric about a second centerline equally dividing the main surface in two and orthogonal to the first centerline.