A method and an apparatus for an exact measurement of the thickness of a plurality of individual semi-conductor and insulation layers and the determination of the centricity/eccentricity of a medium-voltage and/or high-voltage insulated conductor. The individual semi-conductor and insulation layers of the insulated conductor emerging from an extrusion device are penetrated with X-rays in at least two directions laying within a plane orthogonal to the axis of the insulated conductor. On carrying out a line-scan intensity detection of the X-rays having penetrated the insulated conductor, the thickness of the individual layers as well as the position of the conductor is computed. These values are compared with target ones, and, if necessary, the position of the extruder heads can be corrected accordingly.