During a halt period of time when the introduction of ions is temporarily discontinued to change an objective ion to be selected by a first mass separator in the previous stage, a pulsed voltage having a polarity opposite to that of the ions remaining in a collision cell (4) is applied to an entrance lens electrode (42) and exit lens electrode (44). The ions are pulled by the DC electric field created by this voltage, to be neutralized and removed by colliding with the lens electrodes (42, 44). Thus, the residual ions, which may cause a crosstalk, can be quickly removed from the inner space of the collision cell (4) without contaminating an ion guide (5) to which a radio-frequency is applied. Since no radio-frequency voltage is applied to the lens electrodes (42, 44), the circuit for applying the pulsed voltage can have a simple configuration. Thus, the cost increase is suppressed.