amplifier circuit 26 and captured in a micro ammeter 27 to perform monitoring for example a farady cup in which the ion is received by a cup shaped electrode to measure an amount of generated secondary electron a channeltron in which the electrode is not independently formed but constitutes a high resistance pipe a micro channeltron including channeltrons having diameters range from 10 to 20 micrometers and arrayed in plate or a photomultiplier in which light is converted into a photoelectron by a photoelectric surface to amplify the generated secondary electron can be used as the ion detector the mass spectrometer includes an axis adjusting mechanism 30 on the ion traveling axis connecting a center axis of an ion beam outgoing port of the ion source 1 and a center axis of an incident port of the detector 20 such that center axes of the fine holes of the first orifice 3 the second orifice 6 and the third orifice 9 are aligned with one another therefore in the mass spectrometer the axis position adjustment can be performed at a micrometer level the components such as the octopole 8 and the quadrupole mass separator 11 which are disposed between the ion source 1 and the detector 20 can be adjusted by an axis adjusting mechanism not illustrated for the octopole.