The present invention provides a slot antenna in which the radiation toward the zenith direction is not obstructed although it is arranged on an upper side of a ground conducting plate. The antenna device 11 operates as the slot antenna by providing a slot 14 in an upper plate portion 13 of a shield case 12. The shield case 12 is provided on a ground conducting plate 20. The slot 14 is composed of a first aperture 15 extending in a straight line, a second aperture 16 communicating with one end of the longitudinal direction of the first aperture 15, and a third aperture 17 communicating with the other end of the longitudinal direction of the aperture 15. The apertures 16 and 17 have the same triangular shapes which are point-symmetrical to the center of the aperture 15. When the power is fed by a feeding pin 18 to excite the slot 14, the directions of the electric fields generated at the apertures 16 and 17 are inclined to the direction of the electric field generated at the aperture 15 and the electric fields of the apertures 16 and 17 cancel the electric field of the aperture 15.