A mass spectrometer is provided including: a collision chamber of generating fragment ions by superimposingly applying an AC voltage and a first DC voltage between linear multipolar electrodes, and accelerating the fragment ions by applying a second DC voltage between a front stage electrode and a later stage electrode; a mass spectrometer unit of carrying out mass separation of the fragment ions; and a control unit of determining the second DC voltage based on the mass-to-charge ratios such that the rates of the fragment ions in the collision chamber become equal regardless of the mass-to-charge ratios. Herein, the control unit increases the second DC voltage as the mass-to-charge ratios selected by the mass spectrometer unit become larger. This allows the mass window to be wider even when a DC electric field is generated in order to solve a crosstalk drawback, in the movement direction of the molecular ions.