A mass spectrometer has a uniform magnetic field and an electric field. Further, the spectrometer is equipped with three spaced electrostatic quadrupole lenses. The deflection angle .phi..sub.m of the beam in the magnetic field, the radius r.sub.m of the circle described by the beam in the magnetic field, the deflection angle .phi..sub.e of the beam in the electric field, and the radius r.sub.e of the circle described by the beam in the electric field are so set as to satisfy the following relations:82.degree..ltoreq..phi..sub.e .ltoreq.88.degree.;39.degree..ltoreq..phi..sub.m .ltoreq.41.degree.0.715.ltoreq.r.sub.e /r.sub.m .ltoreq.0.755.The spectrometer can measure masses over a wide range even if it is built as a small-sized instrument. Thus, the instrument is able to measure molecular ions of large masses.