a mass spectrometer possessing both high resolution and durability in a simple , compact structure compared to mass spectrometers of the related art , and characterized in possessing a linear ion trap unit containing a multipolar rod electrode including rod electrodes having fine orifices to allow passage of electrons or ions ; a mechanism to move the ions inside the linear ion trap unit along the axis of the multipolar rod electrode ; and a detector to selectively detect by mass , ions ejected from the linear ion trap unit .