PURPOSE: To obtain a high power conversion efficiency by providing a Fabry- Perot resonator comprising a mirror with plural slot holes arranged in parallel and a mirror with plural grooves arranged in parallel to the oscillation tube.

CONSTITUTION: A Fabry-Perot resonator 11 consists of mirrors 20, 21. Parallel grooves 24 are formed in a reflecting face of the mirror 20 and an electron beam passing hole 22 is formed in the grooves 24 placed in the middle. Moreover, the parallel slot holes 23 are arranged at an equal interval in the reflecting face of the mirror 21. An incident electromagnetic wave is reflected in the mirror 21, and then in the grooves of the mirror 20 anisotropically and this anisotropical reflecting wave and the reflecting wave from the mirror 21 are resonated and the electromagnetic wave is outputted through the slot holes of the mirror 21. Thus, since the electromagnetic wave is propagated in the direction of the resonator axis without causing effect on the oscillation mode, a high power conversion efficiency is obtained.