A voltage applied to an exit gate electrode forming a potential barrier and temporarily trapping ions within the inner space of the ion guide is higher than a voltage at an ion guide's exit end. A higher voltage is applied to the exit gate electrode for a lower m/z value of the measurement target ion, to push back the ion which has slowly moved along a potential gradient and reached the exit end of the ion guide. An ion having a lower m/z value is more likely to be located in a farther region from the exit end and forced to travel a longer distance when voltage applied to the exit gate electrode is lowered. A lower m/z value also means a higher travelling speed toward the orthogonal accelerator, whereby m/z dependency of the time required for travel from the ion guide to the orthogonal accelerator eventually becomes low.