To obtain a more favorable radiation pattern even in a case of arraying a plurality of antenna elements.
An antenna device includes a dielectric substrate, a plurality of antenna elements that disposed along a first direction and respectively transmits or receives a first wireless signal and a second wireless signal having different polarization directions from one another, and a ground plate provided with a long slot to extend in a second direction in a region corresponding to a region between first and second antenna elements next to each other, and a length L in the second direction of the slop satisfies a conditional expression below where a wavelength of the wireless signal is λ0, a relative dielectric constant of the dielectric substrate is εr1, and a relative dielectric constant of a dielectric located on an opposite side of the dielectric substrate with respect to the ground plate is εr2. [ Math . ⁢ 1 ] L > λ g 2 , λ g = λ 0 ( ɛ r ⁢ ⁢ 1 + ɛ r ⁢ ⁢ 2 ) ⁢ / ⁢ 2