PURPOSE: To achieve high resolution and high mass spectrometric characteristic at low cost by improving an interface system lens of a tandem type mass spectrograph.

CONSTITUTION: A mass spectrograph of fore stage (an ion source slit 2, electrostatic quadrupole lenses 3a, 3b, 3c, a sector magnetic field 4, a sector electric field 5, a collector slit 6) and a mass spectrograph of rear stage (an ion source slit 8, electrostatic quadrupole lenses 3a', 3b', 3c', a sector magnetic field 4', a sector electric field 5', a collector slit 6') are connected in series between an ion source 1 and a detector 9, and a pair of half plate lenses 7a, 7b is provided on the connection part. The lenses 7a, 7b have the converging characteristic in the horizontal direction of a trajectory plane and the deflectional function for adjusting axial deviation, and has no lens operation in the direction perpendicular to the trajectory plane.