Radiofrequency method for measurement of interface position between two dielectric media that are non-mixed liquids in particular in a reservoir is considered. The method provides independence of measurement results on dielectric permittivity values of both media. Three sections of TEM (Transverse ElectroMagnetic) transmission lines, of coaxial lines in particular that are located vertically in a reservoir are used for realization of the method. Each of these sections has a horizontal part at its lower end. These horizontal parts have different lengths. Resonant frequencies of electromagnetic oscillations of these sections are measured and then are jointly processed according to formulas inherent to this measurement method in order to determine interface position between two dielectric media. The device for realization of the method is considered. It contains three sections of coaxial transmission lines that are done as a set of coaxially placed inner conductor and three hollow cylindrical conductors. The first section of the coaxial line is done by inner conductor and inner surface of inner cylindrical conductor. The second section of the coaxial line is done by outer surface of inner cylindrical conductor and inner surface of middle cylindrical conductor. The third section of the coaxial line is done by outer surface of middle cylindrical conductor and inner surface of outer cylindrical conductor.