Ion beams are efficiently extracted with an accelerator that includes a circular vacuum container including a pair of circular return yokes facing each other. Six magnetic poles are radially disposed from the injection electrode at the periphery thereof in the return yoke. Six recessions are disposed alternately with the respective magnetic poles in the circumferential direction of the return yoke. In the vacuum container, a concentric trajectory region, in which multiple beam turning trajectories centered around the injection electrode are present, is formed, and an eccentric trajectory region, in which multiple beam turning trajectories eccentric from the injection electrode are present, is formed around the region. In the eccentric trajectory region, the beam turning trajectories are dense between the injection electrode and the inlet of the beam extraction path. Gaps between the beam turning trajectories are wide in a direction 180° opposite to the inlet of the beam extraction path.