To replace simply an impedance converter, and to guarantee surely sealing of a transition part from an electromagnetic horn 3 to the impedance converter.

This filling level measuring device operated by the radar principle, for measuring advantageously the filling level of a medium in a vessel 1 is equipped with a microwave generator for generating a microwave signal, a waveguide 2 for guiding the microwave signal, the electromagnetic horn 3 for working as a transmission antenna and/or a reception antenna, the impedance converter 4, and a connection flange 5. In this case, the electromagnetic horn 3 is mounted on the side facing to the medium of the connection flange 5, and the impedance converter 4 is thrust at least partially into an opening of the electromagnetic horn. A seal plate formed integrally with the impedance converter 4 is arranged between the electromagnetic horn 3 and the connection flange 5.