[en] In an electromagnet device disposed in a particle accelerator, since a magnetic pole positioning operation has been extremely troublesome, and the position can not be changed easily, an effective countermeasure has been demanded. The device comprises an electromagnet main body in which magnetic poles and coils are disposed at the inside of an annular york and an orbit of charged particles is formed at the axis position, and a support mechanism for supporting the electromagnet main body and determining the position of the magnetic poles by rotating the axial line as a center. The support mechanism comprises at least two rotatable rollers disposed in parallel with the axial line of the york and supporting the lower portion of the york from both sides, and a rotationally driving source for rotating the york by at least one of the rollers. The york is rotated by the support mechanism by a required angle, thereby enabling to position the magnetic poles at the circumference of the orbit easily and at high accuracy. Further, the position can be changed freely depending on requirements. (N.H.)