To improve a radiation pattern using a soft boundary wall both of the electric field reflection coefficient and the magnetic field reflection coefficient of which are a specific value as a reflection board so as to reduce current flowing in a radial direction of the reflection board.

Both the electric field reflection coefficient and the magnetic field reflection coefficient of a reflection board are set to -1. A horn antenna 3 is made of metal and connected with a power feeding part with a flange 4, and electromagnetic waves are radiated from an opening part 3a. A reflection board 10 has a circular hole part, which a horn antenna 3 is put through, in its center and consists of a dielectric substrate 11, a metallic plate formed on the rear surface of the substrate 11 and a large number of metallic strip lines 13 formed on the front surface of the substrate 11. The large number of metallic strip lines 13, formed on the front surface of the substrate 11, are formed coaxial with the centers of respective opening parts 3a as their common center. The thickness and the dielectric constant of the substrate 11 are selected, so that an incident wave and a reflected wave may have antiphases from each other on the front surface of the board 10.