This disclosure provides a wideband antenna including a feed line, a ground conductor plate and a radiating conductor element connected to the feed line and facing the ground conductor plate at a distance from the ground conductor plate. A parasitic conductor element is provided on a side opposite to the ground conductor plate as viewed from the radiating conductor plate and is insulated from these plates. A coupling adjusting conductor plate is positioned between the radiating conductor element and the parasitic conductor element, is configured to adjust an amount of coupling between them, overlaps an area where the radiating conductor element and the parasitic conductor element overlap, and straddles the radiating conductor element in a direction orthogonal to the direction of a current I that flows therein. Both end sides of the coupling adjusting conductor plate are electrically connected to the ground conductor plate via via-holes.