An antenna device includes: an antenna unit provided as a conductive pattern on a substrate; and a propagation preventive unit which is provided adjacent to the antenna unit and prevents propagation of radiation waves of the antenna unit along the substrate, and the propagation preventive unit is provided as a conductive pattern on the substrate, has plural patches arranged in a prescribed pattern, and has, at an end on a side of the antenna unit, a stepped structure in which a distance from a position of the antenna unit to one of the patches closest to the position of the antenna unit varies by a prescribed interval every time the position in an extension direction of a feed line of the antenna unit is changed by a prescribed distance.