Four main rod electrodes included in a main electrode section are disposed in a rotationally symmetric manner around an ion optical axis. Among four pre-rod electrodes included in a pre-electrode section disposed in front of the main electrode section, two are in contact with a circle of a radius r0, whereas the other two are disposed to be in contact with a circle of a radius R0 larger than r0, resulting in rotational asymmetry around the ion optical axis. Accordingly, a shape of acceptance on an x-y plane regarding positions of ions in the pre-electrode section becomes elliptical. This allows the shape of the acceptance to become gradually flat as the ions travel along the ion optical axis, reducing a mismatch between emittance of incoming ions and the acceptance on a receiving side, and relieving ion loss during ion introduction.