PURPOSE: To prevent the injection of ions which are not aimed at into an ion detector in a SIM measurement by continuously changing both DC and high frequency voltages applied to a quadrupole, in averting their changing loci from an ordinary mass scanning loci when the detection of one specified mass ions is transferred to the detection of the next specified mass ions.

CONSTITUTION: In the selective ion monitoring measurement (SIM) when a DC voltage U and a high frequency voltage V to be applied to a quadrupole of a quadrupole mass analyzer are changed from those for a certain mass to those for another mass, the ratio between U and V is arranged to be averted from a mass scanning locus. When the applied voltages to the quadrupole are changed from P1 to P2 they are changed along the voltage loci K which take a long way around the vertex of a stable area triangle of ions with different mass from the aimed value. The mass of ions to be detected can be thus changed from M1 to M3 without the injection of M2 mass ions into the detector.