PURPOSE: To steadily generate a high-power electromagnetic beam with high efficiency by providing cooling agent passages among slots so that a mirror with a train of slots can be uniformly cooled.

CONSTITUTION: Since a mirror 9 generates Joule's heat by the reflection of electromagnetic waves, a gyrotron must be cooled to be steadily operated. Therefore, an aqueduct jacket is provided as shown by the r-θ cross section diagram of the mirror 9. This aqueduct jacket is constituted of circular passages 1, 2 and radial passages connecting them, and these radial passages are provided among a train of slots 11. The cooling water flowing in through a cooling water inlet enters the upper half (divided by a partition plate) of the circular passage 1 and passes the radial passages (upper half) and enters the circular passage 2. Then, it passes the radial passages (lower half) from the passage 2 and flows into the lower half of the passage 2 and is guided to a cooling water outlet. Accordingly, the mirror 9 is uniformly cooled.