An antenna includes antenna elements provided one by one on the respective sides of a substantially rectangular conductor plate. Each of the antenna elements includes a feeding wire and a split ring conductor having a shape in which a ring is partially cut by a split part. The feeding wire is electrically connected to the split ring conductor and extends in a direction across a region formed inside the split ring conductor. Two antenna elements provided on two arbitrary sides facing each other of the conductor plate among the four antenna elements are each supplied with power through the feeding wire included in each antenna element so as to have substantially the same direction of an electric field in a polarization direction.