Multibeam interferometry technique for investigation self-sustaining and laser-induced MW-discharges in supersonic flows (static pressure 10-50 Torr) and quiescent gas has been developed. The experimental setup contains MW generator, gas-dynamic vacuum chamber, ignition laser and diagnostic system. MW, 1,5 microsecond, X-range pulse generator with output power 200 kW is used to create either the self-sustaining discharge or the laser-induced MW discharge (at the atmospheric pressure). In the last case, Q-switched, 25 nanosecond, 0,2 mJ Ruby laser is used. The temporal evolution of optical density fields for breakdown plasma and thermal well, arising on this place are studied using the high-sensitive intro-chamber Fabry-Perot interferometer. Investigation area of supersonic flow is placed between the interferometer mirrors, which are installed inside the test chamber. In this case the chamber windows are not included in the optical path of the interferometer and it sensitivity for intro-cavity phase objects becomes very high. Intro-chamber plane interferometer Fabry-Perot has fineness about 14, light orifice diameter 86 mm and distance between mirrors 650 mm. Interferometer operates with single - frequency, stabilising (δλ/λ ≤10^-10) He-Ne laser with output power 1 mW . The interference picture is analysing by the high-sensitive gated (exposition time 1-500 μs) CCD camera. These pictures have a high fineness and comparatively good S/N ratio (20-25). It allows to get the sensitivity for interference line shift less than 1/50 of interference band. For thermal well with diameter about 5 mm, arising in flow with static temperature 200K and static pressure 35 Torr, this minimal measuring shift corresponds to temperature changing about 10 K. The heating level for this thermal well at the moment 10 μs after MW discharge is 250±10K. Analysis of interference pictures of wave structures, arising in quiescent gas at the atmospheric pressure at the delays 50-100 μs after laser ignition spark shows, that very deep thermal well exists at the place, were the most effective MW energy inputs is monitoring.