PURPOSE: To improve the analytical sensitivity of a tetrode-type mass spectrograph by installing between an ion source for producing ions and an analyzing part from separating masses, a reduction zoom lens formed by the combination of plural ion-drawing electrodes.

CONSTITUTION: A tetrode-type mass spectrograph 1 is constituted by placing between an ion source 2 and an analyzing part 3, a reduction zoom lens 14 formed by the combination of ion-drawing electrodes 16, 18, 20 and 22. The ion source 2 has an ionization box 4. Around the ionization box 4, a filament 6 for producing thermions and a detector 8 for detecting a current of thermions after ionization are installed. The box 4, the filament 6 and the detector 8 are connected to an ion-production-controlling part 10. The analyzing part 3 has a tetrode electrode 12. The electrodes 16, 18, 20 and 22 have hollow cylindrical shapes and their central axes coincide with the ion-drawing hole 4a of the box 4 and the central axis of the electrode 12. In addition, a voltage-controlling part 24 is connected to the electrodes 16, 18, 20 and 22. Owing to the above constitution, the sensitivity of the whole of the mass spectrograph can be increased without deteriorating its resolution.