An antenna device according to an embodiment includes a dielectric substrate, a housing, and an interference prevention unit. On the top surface side of the dielectric substrate, a plurality of antennas is formed, and on the bottom surface side, a ground is formed, each as a conductive thin film pattern, respectively. The housing is formed of a conductive material, and formed to have a shape configured to function as a waveguide, and the top surface side of the housing is bonded to the bottom surface side of the dielectric substrate. The interference prevention unit is formed between the neighboring antennas to include at least a groove provided on the top surface side of the housing and a slit provided on the ground in the portion corresponding to the groove.