An apparatus for determining and/or monitoring at least one process variable of a medium. The apparatus includes: at least one probe unit; and at least one electronics unit, which supplies the probe unit with an operating signal and which receives from the probe unit a received signal. The probe unit includes at least one probe electrode and at least one guard electrode; and the probe electrode and the guard electrode are surrounded, at least partially, by at least one insulating unit. The the insulating unit has, in the region of the guard electrode, a smaller thickness than in the region of the probe electrode.