A curved ion guide includes four curved rod electrodes arranged around a curved central axis, two deflecting auxiliary electrodes which face each other across the axis, and two focusing auxiliary electrodes which are located on a curved surface orthogonal to the plane P and including the axis and which face each other across the axis. Ions are focused by the effect of an electric field created by radio-frequency voltages applied to the curved rod electrodes, and a deflecting electric field having the effect of curving ions along the axis is created by direct-current voltages applied to the deflecting auxiliary electrodes. Furthermore, a focusing direct-current electric field having the effect of pushing ions from the vicinity of the focusing auxiliary electrodes toward the axis is created by a direct-current voltage having the same polarity as that of the ions and applied to the focusing auxiliary electrodes.