A high frequency ion guide (20) for transporting ions to a subsequent stage while converging the ions by a high frequency electric field includes eight rod electrodes (21 to 28) arranged to surround an ion optical axis (C). Each of the rod electrodes (21 to 28) is arranged to be inclined toward the ion optical axis (C) so that a radius r2 of an inscribed circle (29b) at an end face on the ion emitting side is larger than a radius r1 of an inscribed circle (29a) at an end face on the ion incident side. Accordingly, a gradient of pseudo-potential size or depth is formed in the travelling direction of ions in a space surrounded by the rod electrodes (21 to 28) to accelerate ions according to the gradient.Therefore, ions are restrained from decelerating and delay or stop of ions is prevented even when a gas pressure is relatively high and ions often impinge gas.
