High accuracy measurements of central frequencies of molecular spectral lines are of great importance for an extensive search of spatial-temporal variations of the fundamental constants, which is based on the observations of atomic and molecular spectral lines in the interstellar medium (see for example [1–3]). It was found that transition frequencies of some molecules with intramolecular large amplitude motions have extremely high sensitivity to a possible variations of the proton-to-electron mass ratio μ=mp/me with methanol molecule being at the moment the most promising probe of such variations [2,4,5]. In order to reach a new level in studying of possible variations of μ=mp/me it is necessary to provide relative accuracy of laboratory transition frequency measurements as high as 10− 8−10−9. This value corresponds to absolute experimental uncertainty of laboratory measurements better than 1 kHz in the millimeter-wave range. It should be mentioned that similar accuracy is also required for detailed studies of systematic velocity fields in star-forming regions [6]. That is why the main purpose of this work is to improve accuracy of line frequency measurements up to sub kilohertz level and then to measure frequencies of some methanol lines which were used to probe possible proton-to-electron mass ratio variation [2,3].