An antenna device (2, 102, 202, 302, 502, 602, 702), comprising: a ground plate (4, 104, 204, 304, 504, 704) having substantially orthogonal side portions (12, 14, 212, 214, 312, 412) to which first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742), each including a radiating element (26, 46, 226, 246, 326, 346, 626) and a ground terminal (28, 48, 224, 248, 328, 348, 628), are respectively connected, with one of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742) being powered, wherein for each said antenna the radiating element (26, 46, 226, 246, 326, 346, 626) extends in a substantially parallel direction to the side portion (12, 14, 212, 214, 312, 412) to which that antenna is connected and the ground terminal (28, 48, 224, 248, 328, 348, 628) extends in a substantially perpendicular direction to that side portion (12, 14, 212, 214, 312, 412), the ground plate (4, 104, 204, 304, 504, 704) including: a first slit (62-1, 262-1, 362, 762) extending from a portion where the ground terminal (28, 48, 224, 248, 328, 348, 628) of one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742) is connected to the ground plate (4, 104, 204, 304, 504, 704), in a direction parallel to the ground terminal (28, 48, 224, 248, 328, 348, 628), and a second slit (62-2, 262-2, 363, 763) extending from the tip side of the first slit (62-1, 262-1, 362, 762) in a direction parallel to the radiating element (26, 46, 226, 246, 326, 346, 626) of the one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742). a ground plate (4, 104, 204, 304, 504, 704) having substantially orthogonal side portions (12, 14, 212, 214, 312, 412) to which first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742), each including a radiating element (26, 46, 226, 246, 326, 346, 626) and a ground terminal (28, 48, 224, 248, 328, 348, 628), are respectively connected, with one of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742) being powered, wherein for each said antenna the radiating element (26, 46, 226, 246, 326, 346, 626) extends in a substantially parallel direction to the side portion (12, 14, 212, 214, 312, 412) to which that antenna is connected and the ground terminal (28, 48, 224, 248, 328, 348, 628) extends in a substantially perpendicular direction to that side portion (12, 14, 212, 214, 312, 412), the ground plate (4, 104, 204, 304, 504, 704) including: a first slit (62-1, 262-1, 362, 762) extending from a portion where the ground terminal (28, 48, 224, 248, 328, 348, 628) of one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742) is connected to the ground plate (4, 104, 204, 304, 504, 704), in a direction parallel to the ground terminal (28, 48, 224, 248, 328, 348, 628), and a second slit (62-2, 262-2, 363, 763) extending from the tip side of the first slit (62-1, 262-1, 362, 762) in a direction parallel to the radiating element (26, 46, 226, 246, 326, 346, 626) of the one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742). a first slit (62-1, 262-1, 362, 762) extending from a portion where the ground terminal (28, 48, 224, 248, 328, 348, 628) of one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742) is connected to the ground plate (4, 104, 204, 304, 504, 704), in a direction parallel to the ground terminal (28, 48, 224, 248, 328, 348, 628), and a second slit (62-2, 262-2, 363, 763) extending from the tip side of the first slit (62-1, 262-1, 362, 762) in a direction parallel to the radiating element (26, 46, 226, 246, 326, 346, 626) of the one antenna of the first and second antennas (22, 42, 222, 242, 322, 342, 422, 542, 622, 722, 742).