Abstract: A measuring device for a machine tool comprises at least one sensor mounted on a supporting arm which is movable with the help of a carriage. To perform an exact measurement of the distance between two diametrically opposed points on a work piece, the supporting arm with the sensor is slidably and/or pivotally mounted on the carriage to displace the sensor in a transverse direction over the machine tool turning center. A folding in and out of the supporting arm which is mounted on a holder on the carriage occurs during insertion into and withdrawal from a storage pocket of a tool magazine. Air cleaning means and cooling means for the sensor and the workpiece are provided. In another embodiment of the measuring device the supporting arm can be approximately U-shaped mounted on a shaft with a sensor on the end of each U-leg for a two point workpiece measurement, can be slid along the holder for the supporting arm and is pivotable about 90 degrees.