PURPOSE: To maintain the betatron frequency in acceleration of charged particles to be unvaries and thereby to enable stable circulation thereof by a method wherein four-pole shim coils for eliminating a magnetic field gradient of magnetic poles are provided between the magnetic poles.

CONSTITUTION: Two pairs of four-pole shim coils 4 are provided between magnetic poles 2 and they are so designed as to generate a magnetic field gradient in the reverse direction to the magnetic field gradient generated as a residual magnetic field. This magnetic field gradient generated as the residual magnetic field has a very weak value and the magnetic field gradient generated by the coils 4 needs also to be only a little. A magnetic field between the magnetic poles 2 is expressed by the sum of the residual magnetic field and the magnetic field generated by the coils 4 and has a uniform magnetic field distribution having no magnetic field gradient. Thereby it is made possible to maintain the betatron frequency in acceleration of charged particles to be unvaried and the charged particles can be accelerated stably.