A first rod electrode set has a first center axis, into which ions and air current are introduced. A second rod electrode set has a second center axis at a distance from the first center axis, from which the ions are discharged. A power supply applies voltages to the first rod electrode set and the second rod electrode set. The first rod electrode set and the second rod electrode set have a region where the sets overlap each other in the longitudinal direction, and form a single multipole ion guide by being combined to each other in the region. Different offset DC voltages are applied to the first rod electrode set and the second rod electrode set, respectively, and a DC potential for moving the ions to the second rod electrode set in the region is formed, the ions having been guided by the first rod electrode set.