Abstract: An apparatus for measuring the level of the interface between two different media in a reservoir comprises a vibratory element adapted to contact both media on all sides, and an amplifier circuit which together with the vibratory element forms an oscillator circuit in which the vibratory element is the frequency determining part. The vibratory element makes flexural vibrations at a resonance frequency. The apparatus further comprises a processing unit for measuring the momentary resonance frequency of the vibratory element and for deriving the level of the interface with respect to the vibratory element from said momentary frequency by means of the predetermined dependency of the resonance frequency on the depth of immersion of the vibratory element in the lower medium. The vibratory element is made as a plate-like element with a very small size with respect to the height of the reservoir.