A planar array antenna comprises a powered antenna element and an adjacent passive element which are microstrip-line type ones and disposed on one principal surface of a dielectric substrate; and a feeding system for feeding high frequency power to the powered antenna element. The powered antenna element and a passive element disposed ahead of the powered antenna element constitute a powered element pair, and the adjacent passive element and a passive antenna element disposed ahead of the adjacent passive element constitute a passive element pair. The passive element pair is disposed so that it adjoins said powered element pair in an electric field direction or a magnetic field direction of radio wave emitted from the powered antenna element.