An antenna device includes a ground electrode, a feed element, and a parasitic element. The ground electrode has a substantially non-square rectangular plane shape that includes a first side extending in a first direction and a second side extending in a second direction orthogonal to the first direction. The feed element has a substantially rectangular plane shape and is formed in such a way that each side of the feed element becomes parallel to the first direction or the second direction. The parasitic element is formed in such a manner as to face a side of the feed element parallel to the first side. The feed element is configured to radiate a first polarized wave that excites in the first direction and a second polarized wave that excites in the second direction. The length of the first side is longer than the length of the second side.