PURPOSE: To reduce the intensity of a couter magnetic field, which is generated by an eddy current at the rise time of a deflecting magnetic field, so as to easily generate a magnetic field corresponding to beam energy.

CONSTITUTION: An iron core 10 in a block structure is split into the upper part iron core 10a and the lower part iron core 10b, the upper part iron core 10a having the upper end left and the lower part iron core 10b having the lower end left so that plural slits 11a, 11b are provided in parallel to the direction of a magnetic field and in parallel to a plane perpendicular to the direciton of charged particles to be deflected. In this way, the intensity of a counter magnetic field which is generated by an eddy current at the rise time of a deflecting magnetic field is reduced so as to easily generate and conrol a magnetic field corresponding to beam energy.