PURPOSE: To prevent the reduction of the analysis work efficiency of a sample when the ion beam quantity from an ion source is reduced and prevent the breakage of the sample when the ion beam quantity from the ion source becomes too large.

CONSTITUTION: In an ion beam analyzer provided with objective slit plates 2ax1, 2ax2, 2ay1, 2ay2 adjustable for the widths of slit gaps where an ion beam passes, the ion current value to flow in a sample S when the ion beam of the quantity to be radiated is radiated to the sample S is set as the sample excitation current preset value in advance. The ion current value flowing in the sample S radiated by the ion beam is measured by an ammeter 11 during the radiation of the ion beam, the difference between the sample excitation current measured value and the sample excitation current preset value is obtained, and the slit gap widths of the objective slit plates are automatically adjusted by a motor driving circuit 2b in response to the difference value to eliminate the difference.