A magnet device that includes upper and lower disk-shaped return yokes, a pair of upper magnetic pole and lower magnetic pole respectively fixed to a disk-shaped surface of the upper return yoke and a disk-shaped surface of the lower return yoke, in which a space to circulate and accelerate an ion beam is formed between the upper magnetic pole and the lower magnetic pole. The upper magnetic pole and the lower magnetic pole have a plurality of concave and convex parts along a track along which the ion beam circulates, are plane-symmetrical with respect to a horizontal symmetry plane formed by the track along which an ion beam circulates, and are plane-symmetrical to one of the vertical planes vertical to the horizontal symmetry plane. Also, the magnetic pole intervals between the concave parts of the upper magnetic pole and the lower magnetic pole are different from each other.