PURPOSE: To improve thermal characteristics and starting property by forming a turning body located in a microwave cavity in which an electrodeless lamp is disposed, in the configuration of a turning surface protruding toward the net plane side and having its rotatory axis in common with the turning body at the lamp rear location.

CONSTITUTION: A microwave cavity resonator 6 with an electrodeless lamp 4 is formed using a net plane 3 and two turning bodies 1a and 1b. Besides, the turning body 1a is set so that the light from the lamp 4 can be emitted outward by a single reflection and the turning body 1b is protruded inside the turning body 1a. Then a feed port 9 is arranged. The light from the lamp 4 is incident on the turning body 1b, reflected by the turning body 1a, and emitted outward. As a result, optical alignment can be controlled over a comparatively wide range, the feed port 9 be provided in the proximity of the lamp 4 while the thermally allowable size of the lamp is being kept, and an optical loss can be reduced without deteriorating its starting property.