The computations and experimental investigations thus carried out show that the use of an external resonator as a frequency discriminator of a laser provides it with a low-inertia automatic frequency control system. The design developed for a high-Q optical resonator provides it with a long-term frequency stability. The application of this model in a laser automatic frequency control system serves to obtain under complex conditions a short-term laser frequency stability better than 1·10−8 and a long-term stability not worse than 4·10−8 for 15 min.