We describe a unique new-generation laboratory facility for studying dielectric parameters of gases and condensed media, as well as reflectivity of surfaces (reflection loss) in the frequency range 100–500 GHz and pressure interval from 10−3 Torr to the atmospheric pressure at temperatures of 4 to 370 K. The Fabry–Perot resonators with Q-factors of about 106, in which the studied gas, dielectric, or reflector are located, are the measuring elements of the facility. The backward-wave oscillator stabilized by the wideband phased-lock loop is the radiation source. Using this facility, we were able, in particular, to confirm the presence of water dimers in the atmosphere and study some materials for the reflectors of the “Millimetron” space observatory.