To implement an antenna device capable of further reducing an influence of proximity to a metal and feeding power to an antenna element in a more suitable manner.
An antenna device includes: a substantially-flat-plate-shaped dielectric substrate; a metal base plate arranged on a first surface of the dielectric substrate; substantially-flat-plate-shaped first and second antenna elements arranged on a second surface of the dielectric substrate that is opposite to the first surface and on an opposite side of the dielectric substrate from the metal base plate so that a slit is formed; a first feeding pin that feeds power to the first antenna element; and a second feeding pin that feeds power to the second antenna element, in which a phase difference between feeding signals supplied to the first and second feeding pins, respectively, is approximately 180 degrees.