PURPOSE: To obtain an electron impact type ion source device of fine diameter with high brightness.

CONSTITUTION: A filament 1, electron accelerating electrode 2, electron reflecting electrode 4 and an ion drawing electrode 5, opposed to a central small hole 41 of the electron reflecting electrode, are arranged on an axis of an ion source device to form a magnetic field H so as to concentrate toward the central small hole of the above-mentioned electron reflecting electrode from the filament. In this way, an ion source of high brightness is obtained by concentrating also an ionizing electron cloud and plasma in the central hole 41 of the electron reflecting electrode.