Within an intermediate vacuum chamber next to an ionization chamber maintained at atmospheric pressure, an electrode group of a radio-frequency carpet composed of a plurality of concentrically arranged ring electrodes is placed before a skimmer, with its central axis coinciding with that of an ion-passing hole. Each ring electrode has a circular radial sectional shape. Radio-frequency voltages with mutually inverted phases are applied to the ring electrodes neighboring each other in the radial direction. Additionally, a different level of direct-current voltage is applied to each ring electrode to create a potential which is sloped downward from the outer ring electrode to the inner ring electrode. The circular cross section of the electrode produces a steep pseudo-potential near the electrode and thereby increases the repulsive force which acts on the ions to repel them from the electrode.