PURPOSE: To oscillate and amplify radiation laser light of short wavelength with high efficiency and high gain, by arranging direction control means in a polygonal type.

CONSTITUTION: A pulsed electron beam accelerated at specified time intervals by an RF electron accelerator 6 passes in succession a reflecting mirror 2' of a direction control means 3 in the incidence end, and is sent into the orbit of a resonator 5. Said beam is made to meander by the periodically changing magnetic field of wiggler magnets 4, and radiation laser light is generated as the result of mutual reaction with the electromagnetic field. Since the velocity of said light is a little larger than that of electrons, the timing is adjusted by adjusting the magnetic force by changing the exciting current of a deflecting electromagnet 1, in order that the radiation laser light and the electron beam simultaneously reach the next wiggler magnet 4. Thus the radiation laser light which is oscillated and amplified travels the adjacent mutual reaction regions in succession together with the electron beam, and oscillation and amplification are repeated. Then the laser light returns the incidence end. The output light is led out to the outside by the reflecting mirror 2' of the incidence end.