In order to simplify the calibration of mass spectra, a mass marker has been designed for the AEI mass spectrometer MS2H which, with slight modifications, can be applied to any other magnetically scanned spectrometer. During the scanning of the spectrometer, the apparatus operates the marker pen of the recorder so that calibration markings are registered during recording of the spectrum. The marker is based on accurate determination of the current through the deflection magnet which is used for the magnetic scanning in the spectrometer. The influence of the hysteresis effect is eliminated by automatic programming of the magnet current before each measurement. When the marker is used with an AEI mass spectrometer MS2H, spectra can be calibrated at every fifth m/e in the range 60-535 m/e, provided the ion-accelerating voltage is 2000 v. The accuracy is to 0.3 m/e.