Disclosed here is a cyclotron having a beam phase selector capable of controlling phase widths of beams and improving beam permeability for increasing beam current. The cyclotron contains an acceleration voltage applying section and a beam blocking section, at least any one of the two sections has a movable structure. While a particle is passing across a gap between dee electrodes, the acceleration voltage applying section applies RF acceleration voltage to the particle, and further applies RF acceleration voltage having a phase different from the phase of previously applied RF acceleration voltage. The beam blocking section blocks undesired particles. Preferably, the acceleration voltage applying section at least has an electrode having an opening in a direction of the core of the cyclotron. Also preferably, operations on phase-width control can be performed outside the cyclotron, with vacuum condition in the cyclotron maintained.
