PURPOSE: To emit electromagnetic waves oscillated in a resonator to outside thereof effectively by furnishing a straight string of slots at either or both of the two mirrors constituting an optical resonator.

CONSTITUTION: A resonator 2 is formed from a spherical mirror 1 and a flat mirror 1' facing each other. An electron beam 5 causes oscillation of a radiational wave in this resonator 2 with the X-X' axis as center. The electric field of the oscillated electromagnetic wave lies in the plane perpendicular to the paper in such a way as right across the Y-Y' axis. Accordingly the electromagnetic wave oscillated in the resonator 2 with the output mirror 1' as a partial penetrating mirror can be emitted in the same mode as within the resonator 2 by furnishing the mirror 1' with a string of slots consisting of straight slots 9 parallel to the Y-Y' axis at a spacing d narrower than the wave-length. Thus the electromagnetic wave can be emitted effectively to outside, that enables transmission till, for ex., nuclear fusion plasma while reflected by a plurality of spherical mirrors.