To suppress the generation of reflected waves in a TEM mode without having any adverse effect on radiative characteristics with a simple constitution by electrically connecting a pair of ground conductors so that the periphery of a slot can be surrounded within a specific range, and providing shielding members for electrically shielding the inner side and outer side.

A pair of ground conductors 1 and 5 are electrically connected so that the whole periphery of a slot 6 can be surrounded within a range from a position at which a first central conductor 3 and the slot 6 are overlapped to not less than 1/4 and not more than 1/2 of the wavelength of radio waves transmitted and received through the first central conductor 3, and plural through-holes 9 for electrically shielding the inner side and the outer side are formed. A thin cavity using the slot 6 as an opening is formed of the through- holes 9 and the ground conductors 1 and 5. This cavity is formed in length which is not less than 1/4 and not more than 1/2 of the wavelength of the transmitted and received radio waves so that reflected waves generated from the slot 6 can be reflected on the side faces of the cavity, and provided with in- phases on the slot 6 so as to prevented from being propagated.