To provide an accelerator that easily provides a space for placing equipment incorporated into an accelerator magnet, and that has a dense region with small turn separations of beams and a sparse region with large turn separations of the beams in different positions in the beam orbit direction. A pair of magnetic poles (8, 9) has a depression structure of a plurality of depression and projection structures, in a position intersecting with a vertical plane (3). A boundary surface (41, 44, 45, 48) between the depression structure (21, 23) placed in a position intersecting with the vertical plane (3) and a projection structure (31, 32, 33, 34) adjacent to the depression structure has unanimously either a projection shape or a depression shape with respect to the vertical plane (3).
