A tandem Fourier transform ion cyclotron resonance mass spectrometer is provided. In the mass spectrometer, the ions selected by a FT-ICR mass analyzer, which can perform an ion selection process and a mass measurement process with a time interval between the processes, are transmitted through an ion guide to a collision cell, which is located a predetermined distance from the FT-ICR mass analyzer, to split into fragment ions. The fragment ions are transmitted to the FT-ICR mass analyzer that measures the mass of the fragment ions. The fragment ions are generated in the collision cell 60 established separately from the FT-ICR mass analyzer 40 according to the mass spectrometer. Accordingly, It can solve various problems (e.g., the radius reduction of cyclotron motion of colliding ions, or the removal of periphery gas after generating the fragment ions) occurred in a tandem mass spectrometer using a conventional tandem-in-time mass analysis method. Also, a high resolution and with sensitivity measurement can be achieved. Moreover, when a reagent gas instead of a collision gas in the collision cell is injected, the gas phase reaction of the selected ions and the reagent gas can be observed, and the mass of the ions generated in the gas phase reaction can be measured.