PURPOSE: To enable a vacuum value measurement in an ion source without a vacuum meter for exclusive use, by installing an electric current shock type ion source repeller or detecting a current value of an electric current flowing in a correspond unit of the repeller.

CONSTITUTION: Between a repeller 4 and a filament 8, an electric current detector 6 detecting a repeller current Ir and between a grid 14 and the filament 8 an electric current detector 18 detecting an emission current Ie are installed respectively. An electric current flow quantity of the filament 8 is so controlled that the current Ie becomes 10mA assuming a bombard voltage Vb as 200V and a repeller voltage Vr as 10V, then a distinctive dependability is brought to existence between the current Ir at the time when Ar gas is introduced and a vacuum value of an ionizing chamber. Accordingly, by obtaining preliminarily a relation between current Ir and vacuum value, measurement of the vacuum value of the ionizing chamber can be made after detection of the current Ir.