To reduce strain transformation due to thermal expansion in a slot plate by a method, wherein a flat recess part is formed on the lower face of an antenna guide, and a disk-type dielectric plate is mounted in the recess part, and the dielectric plate is made of aluminum nitride.

A flat recess part 4 is formed on the lower face of an antenna guide 2, and the upper face of this antenna guide 2 is connected to a wave guide 5 in which microwaves the guided into the recess part 4. Furthermore, a disk-type dielectric plate 6 made of aluminum nitride ceramics is mounted in the recess part 4 in the antenna guide 2, and a slot plate 7 in which a plurality of slot holes 8 are opened is arranged at the lower face side, and the outer peripheral part is arranged so as to come into close contact with the lower face of the antenna guide 2, and also in this slot plate 7, the outer peripheral part thereof is fitted on the antenna plate 2 with screwing of a plurality of screws.