In an annular microstrip antenna element, an annular radiant conductor plate is mounted on a ground conductor plate via a dielectric layer. A coaxial transmission line is provided, whose external conductor is connected to the ground conductor plate, and whose central conductor is connected to the radiant conductor plate, via a feeding point, with a pair of microstrip lines. The pair of microstrip lines extend from the feeding point to two connecting points on the radiant conductor plate, forming an angle between them like a V shape. By varying the angle, it is possible to adjust the input impedance of the annular microstrip antenna element at the feeding point, so that impedance matching with the coaxial transmission line is achieved. In a radial line antenna system comprising a plurality of the antenna elements, the antenna elements are mounted on concentric circles with a constant space between the adjacent antenna elements, thereby reducing undesirable electromagnetic coupling.