To reduce a transmission loss in a high frequency band such as a submillimeter wave or a millimeter wave, to improve opening efficiency, to impart a small thickness and simple structure and to prevent breakage due to scattered matters, etc., and characteristics variation due to sediments.

In a planar antenna for leaking electromagnetic waves from the surfaces of a plurality of image lines 34 consisting of a dielectric material arranged in parallel to a base board conductor, a nearly flat cover member 40 having a dielectric constant smaller that that of the dielectric material forming the line 34 is fixed under the condition where its one surface side is abutted on the surface of the lines 34.