[en] The detailed discussion is given of a microwave gas spectrometer with Stark modulation constructed our Institute as well as of its application to the study of microwave rotational resonance. The influence of an electric field on the energy levels of molecules having a dipole moment is demonstrated, and applied in the microwave spectrometer with Stark modulation. The separation principle of the spectrometer is presented, as well as its successive sub - units: the microwave source, absorption chamber, receiving circuit, vacuum system, and frequency marker comprising i.a. a frequency stabilizer. Results obtained when measuring the quadrupole structure of the inversion line of ammonia, the dipole moment of the OCS molecule, as well as self-broadening of the inversion line of ammonia and the internal rotation line of methanol for various rotational states, are discussed. (author)