A device for measuring the thickness of slag on the surface of liquid metal contained in an ingot mould comprises a wire made of electrically conductive material and capable of being eliminated under the effect of the heat at the temperature of the slag, the wire having a free end able to be dipped into the slag, a wire feeder capable of displacing the wire so that its free end dips vertically into the slag according to a predetermined trajectory, a wire displacement measurer capable of measuring a distance travelled by the free end of the wire during a time interval between two predetermined events when the latter is displaced under the action of the feeding means, and a wire feeder controller having a molten metal contact detector capable of detecting contact between the free end and the surface of the liquid metal.