There is provided a cyclotron capable of improving the beam efficiency. A cyclotron 1 of the present invention includes: a hollow yoke 3； first and second poles 6 and 7 disposed in the yoke 3； an ion source 2 that generates an ion； a buncher 8 of which at least a part enters the yoke 3 and which adjusts the density of an ion beam R, which is emitted from the ion source 2, in a traveling direction； and an inflector 9 that deflects the ion beam R having passed through the buncher 8 to make the ion beam R incident on a median plane.本發明提供一種能夠提高射束效率之迴旋加速器。本發明之迴旋加速器(1)具備：中空的磁軛(3)；上桿(6)和下桿(7)，配置於磁軛(3)內；離子源(2)，生成離子；集束器(8)，至少一部份進入到磁軛(3)內，且對從離子源(2)送出之離子束(R)的前進方向的密度進行調整；及反曲器(9)，使通過集束器(8)之離子束(R)偏向而射入於正中切面(P)。
