An antenna device comprising a ground conductor plate in which a first opening (2-1, 2-2) is formed; a first waveguide (4-1, 4-2) that is short-circuited to the ground conductor plate in such a way that an input output end thereof is connected to the first opening (2-1, 2-2); and a second waveguide (5-1, 5-2) in which a first input output end thereof is connected to another input output end in the first waveguide (4-1, 4-2), for deflecting a direction of an electric field of an electromagnetic wave supplied from the first input output end or a second input output end thereof in such a way that a direction of an electric field of an electromagnetic wave at the first input output end differs from a direction of an electric field of an electromagnetic wave at the second input output end by 90 degrees; and wherein a second opening (3-1, 3-2) whose longitudinal direction is perpendicular to that of the first opening (2-1, 2-2) is formed in the ground conductor plate, wherein the antenna device comprises at least one third waveguide (8-1, 8-2) that is short-circuited to the ground conductor plate in such a way that one input output end thereof is connected to the second opening (3-1, 3-2); characterized in that it further comprises at least one taper conductor configured such that electromagnetic waves propagated through space in the first waveguide (4-1, 4-2) are emitted to outside space via the at least one taper conductor, and wherein the at least one taper conductor comprises a plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), wherein the first opening (2-1, 2-2) is connected to at least one of the taper conductors (7-1, 7-2, 7-3) out of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), wherein the second opening (3-1, 3-2) is connected to at least one of the taper conductors (7-1, 7-2, 7-4,7-5) out of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), and wherein each of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6) is provided on a surface of the ground conductor plate and has a taper shape in which a central portion thereof is raised with respect to the ground conductor plate, and wherein each of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6) includes four taper portions (7-1a, 7-2a, 7-3a, 7-4a, 7-5a, 7-6a) extending from the central portion toward orthogonal directions along the ground plate, and wherein each of the first opening (2-1, 2-2) and the second opening (3-1, 3-2) is placed with its center arranged, in a direction perpendicular to its longitudinal direction, between taper portions (7-1a, 7-2a, 7-3a, 7-4a, 7-5a, 7-6a) of two adjacent taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6). a ground conductor plate in which a first opening (2-1, 2-2) is formed; a first waveguide (4-1, 4-2) that is short-circuited to the ground conductor plate in such a way that an input output end thereof is connected to the first opening (2-1, 2-2); and a second waveguide (5-1, 5-2) in which a first input output end thereof is connected to another input output end in the first waveguide (4-1, 4-2), for deflecting a direction of an electric field of an electromagnetic wave supplied from the first input output end or a second input output end thereof in such a way that a direction of an electric field of an electromagnetic wave at the first input output end differs from a direction of an electric field of an electromagnetic wave at the second input output end by 90 degrees; and wherein a second opening (3-1, 3-2) whose longitudinal direction is perpendicular to that of the first opening (2-1, 2-2) is formed in the ground conductor plate, wherein the antenna device comprises at least one third waveguide (8-1, 8-2) that is short-circuited to the ground conductor plate in such a way that one input output end thereof is connected to the second opening (3-1, 3-2); characterized in that it further comprises at least one taper conductor configured such that electromagnetic waves propagated through space in the first waveguide (4-1, 4-2) are emitted to outside space via the at least one taper conductor, and wherein the at least one taper conductor comprises a plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), wherein the first opening (2-1, 2-2) is connected to at least one of the taper conductors (7-1, 7-2, 7-3) out of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), wherein the second opening (3-1, 3-2) is connected to at least one of the taper conductors (7-1, 7-2, 7-4,7-5) out of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6), and wherein each of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6) is provided on a surface of the ground conductor plate and has a taper shape in which a central portion thereof is raised with respect to the ground conductor plate, and wherein each of the plurality of taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6) includes four taper portions (7-1a, 7-2a, 7-3a, 7-4a, 7-5a, 7-6a) extending from the central portion toward orthogonal directions along the ground plate, and wherein each of the first opening (2-1, 2-2) and the second opening (3-1, 3-2) is placed with its center arranged, in a direction perpendicular to its longitudinal direction, between taper portions (7-1a, 7-2a, 7-3a, 7-4a, 7-5a, 7-6a) of two adjacent taper conductors (7-1, 7-2, 7-3, 7-4, 7-5, 7-6).