PURPOSE: To generate an electromagnetic wave of the large power by constituting a resonance part while using an axially symmetrical partially transmissive mirror.

CONSTITUTION: A ring-shaped axially symmetrical partially transmissive mirror 22 is made of copper, while its inner surface 34 has a constant radius of curvature [for instance, a radius of curvature equal to the mirror diameter (2R)] in the r-z section while forming a concave mirror covering all of the periphery and allowing an electromagnetic wave emitted in the radial direction to be reflected, oscillated and amplified. The electromagnetic wave of an electron beam 19 is oscillated and amplified by the partially transmissive mirror 22 for being transmitted in the direction of a mirror 23. The reflection face of the mirror 23 has an oval face 40 having two focal points, namely the center 37 of the partially transmissive mirror 22 and the center 39 of the reflection face 38 of a mirror 24. Similarly, the mirror 24 has two focal points, namely the center 41 of the mirror 23 and the center 43 of the reflection face 42 of the mirror 24, while the electromagnetic wave reflected on the reflection face 38 is reflected on the reflection face 42 for transmitting the electromagnetic wave in the direction of the mirror 25. Also the mirror 25 has the reflection faces 44 and 45 for transmitting the electromagnetic wave in the Z-direction, while the reflection face 45 is formed into an oval rotary face for focusing on the plasma 46.