An apparatus for ascertaining and monitoring fill level of a medium in a container by means of a travel-time measuring method utilizing high-frequency measuring signals. An antenna having an antenna coupling region and an antenna region, or a waveguide having a probe coupling region and a probe measuring region extending into the container is included. Hollow spaces present in the antenna coupling region, in the antenna region, in the probe coupling region and/or in the probe measuring region are filled at least partially with a dielectric filling body. The dielectric filling body has internally at least one, hermetically sealed, reserved volume and the hermetically sealed, reserved volume is so embodied, that the dielectric filling body has a predetermined wave resistance and/or the high-frequency measuring signals have a predetermined propagation characteristic.