PURPOSE: To enable selection between large current ion flow and thin ion flow by arranging a restrictor enable of operation of the hole diameter from the outside of vacuum immediately in the rear of a chip for causing surface ionization in surface ionization type cesium ion source.

CONSTITUTION: Cesium ionizing material 1 is vaporized by means of a filament 6 to cause surface ionization on a porous tungsten chip 2 then the quantity of ion is controlled through a control electrode 3 to take out the cesium ions through a take-out electrode 4. Here, a plate restrictor 5 having plural holes of about 100W1,000μm diameter is arranged immediately in the rear of the chip 2 operatably from the outside of vacuum. The large current ion flow and the thin ion flow are made selectable through selection of hole diameter while to enable shielding of the post stage electrodes 3, 4. Consequently, selection of ion take-out is facilitated while contamination of the electrode due to ionized material can be prevented.