According to one embodiment, an antenna device is provided with a dielectric substrate whose both surfaces are covered by first and second metal films, a via-hole row in which via-holes are arranged in two rows on the dielectric substrate, and a waveguide line is formed by the first and the second metal films, and a slot pair provided in the first metal film. The slot pair has a first slot and a second slot provided so that a slot length direction is oblique to a line direction of the waveguide line. A center of the first slot and a center of the second slot are spaced apart from each other by not less than a half of the shorter one of the slot length of the first slot and the slot length of the second slot along the slot length direction.