The invention relates to a device for monitoring the penetration of an instrument (1) into an anatomical structure, particularly a bone structure. The inventive device comprises a voltage source whichpowers at least two electrodes and a means for measuring the impedance between said electrodes. The invention is characterised in that the electrodes (2, 3) are located on the penetrating instrument(1) such as to present a flush constant contact surface that is dependent on how far the penetrating instrument (1) is inserted into the bone structure.