Abstract: In a mass spectrometer of the magnetic field type for analyzing the mass of ions by causing an ion beam to pass through a narrow slit and then through a deflecting magnetic field to be detected by an ion detector where the intensity of the deflecting magnetic field is varied, plural electrostatic quadrupole lenses are provided between the source slit and the deflecting magnetic field so as to give a converging property to an ion beam passing in a direction vertical to the median plane thereof and to give a diverging property to an ion beam passing in the direction of the radius thereof. This minimizes the gap spacing between the magnetic pole pieces which form the deflecting magnetic field, thereby improving sensitivity and detection as well as accuracy and measurement.