An array antenna device includes: a ground plate that is a flat-plate shaped conductor; an element antenna array in which a plurality of tapered slot antennas are linearly arranged on the ground plate along an electric field direction; and a metal plate provided at both ends or one end of an antenna aperture formed in the element antenna array arranged on the ground plate along the electric field direction, the metal plate having a height from the ground plate higher than a height of the tapered slot antenna.