A mass spectrometer comprising: an ion source for ionizing a sample to create ions; an ion storage portion for storing the created ions and repeatedly ejecting the stored ions as pulsed ions; a first mass analyzer for selecting first desired ions from the pulsed ions ejected from the storage portion based on mass-to-charge ratio; a collision cell for fragmenting some or all of the first desired ions into product ions; a second mass analyzer for selecting second desired ions from the first desired ions and the product ions based on mass-to-charge ratio; a detector for detecting the second desired ions; and a controller for controlling the first and the second mass analyzers (30, 50), said controller arranged to provide control such that the mass-to-charge ratio of the first desired ions selected by the first mass analyzer is changed within a period from an instant at which the preceding pulsed ions finish passing through the first mass analyzer to an instant at which the succeeding pulsed ions enter into the first mass analyzer, and further providing control such that the mass-to-charge ratio of the second desired ions selected by the second mass analyzer is changed within a period from an instant at which the preceding pulsed ions finish passing through the second mass analyzer to an instant at which the succeeding pulsed ions enter into the second mass analyzer, characterized in that the controller is further arranged to control the axial voltage of the first mass analyzer in accordance with the mass-to-charge ratio of the first desired ions such that the time taken for the first desired ions to pass through the first mass analyzer is made substantially constant irrespective of change of the mass-to-charge ratio of the first desired ions, and further controlling the axial voltage of the second mass analyzer in accordance with the mass-to-charge ratio of the second desired ions such that the time taken for the second desired ions to pass through the second mass analyzer is made substantially constant irrespective of change of mass-to-charge ratio of the second desired ions. an ion source for ionizing a sample to create ions; an ion storage portion for storing the created ions and repeatedly ejecting the stored ions as pulsed ions; a first mass analyzer for selecting first desired ions from the pulsed ions ejected from the storage portion based on mass-to-charge ratio; a collision cell for fragmenting some or all of the first desired ions into product ions; a second mass analyzer for selecting second desired ions from the first desired ions and the product ions based on mass-to-charge ratio; a detector for detecting the second desired ions; and a controller for controlling the first and the second mass analyzers (30, 50), said controller arranged to provide control such that the mass-to-charge ratio of the first desired ions selected by the first mass analyzer is changed within a period from an instant at which the preceding pulsed ions finish passing through the first mass analyzer to an instant at which the succeeding pulsed ions enter into the first mass analyzer, and further providing control such that the mass-to-charge ratio of the second desired ions selected by the second mass analyzer is changed within a period from an instant at which the preceding pulsed ions finish passing through the second mass analyzer to an instant at which the succeeding pulsed ions enter into the second mass analyzer, characterized in that the controller is further arranged to control the axial voltage of the first mass analyzer in accordance with the mass-to-charge ratio of the first desired ions such that the time taken for the first desired ions to pass through the first mass analyzer is made substantially constant irrespective of change of the mass-to-charge ratio of the first desired ions, and further controlling the axial voltage of the second mass analyzer in accordance with the mass-to-charge ratio of the second desired ions such that the time taken for the second desired ions to pass through the second mass analyzer is made substantially constant irrespective of change of mass-to-charge ratio of the second desired ions.