Provided is a variable energy and miniaturized accelerator. It is impossible to change the energy of the extraction beam in the related cyclotron or to miniaturize an accelerator in the related synchrotron. The accelerator includes a pair of magnets which form a magnetic field therebetween; an ion source which injects ions between the magnets; an acceleration electrode which accelerates the ions; and a beam extraction path which extracts the ions to the outside. A plurality of ring-shaped beam closed orbits formed by the pair of magnets, in which the ions of different energies respectively circulate, are aggregated on one side. The frequency of the radiofrequency electric field fed to the ions by the acceleration electrode is modulated by the beam closed orbits.