The inside of the collision cell 20 placed in the analysis chamber 10 which is vacuum-evacuated is partitioned into an anterior chamber 23 and a posterior chamber 24 by the partition wall 21 with a communicating aperture 22. The former is used as a dissociation area A1 and the latter as a convergence area A2. Electrodes 27 and 28 for forming a radio-frequency electric field are placed in the chambers 23 and 24, respectively. When a CID gas is supplied into the anterior chamber 23, the CID gas is dispersed inside the anterior chamber 23 and flows into the analysis chamber 10 via the posterior chamber 24. Consequently, the gas pressure in the posterior chamber becomes higher than the gas pressure in the analysis chamber 10, and the gas pressure in the anterior chamber 23 becomes higher than the gas pressure in the posterior chamber 24. Hence, the optimum gas pressure for the precursor ion's dissociation and the ions' convergence by a cooling process can be realized in each of the dissociation area A1 and the convergence area A2. Therefore, it is possible to increase both the efficiency of dissociation and the efficiency of the ion convergence.