This antenna device is provided with a radiation element (10) which is a conductive member electrically connected to an internal conductor of a coaxial cable; and a flat plate part (21) which is a flat-plate-shaped conductive member electrically connected to an external conductor of the coaxial cable. The radiation element is disposed to face the flat plate part at a predetermined distance from the flat plate part or to be erected from the flat plate part. The flat plate part has an axisymmetric shape provided with a narrow width region in which the length, from any one end portion to an end portion positioned on the opposite side to the one end portion with the axis of symmetry therebetween, is set shorter than the half wavelength of a radio wave to be tranceived. In addition, on at leastone of two end portions which are parallel to the axis of symmetry and form the narrow width region, a side wall part (22), which is a conductive member, is provided along the end portion so as to face the direction in which the radiation element is not present when viewed from the flat plate part.