PURPOSE: To measure the vacuum degree of an ion source highly sensitively and highly precisely without using an additional vacuum gauge by installing a microchannel plate in a part of an electrode against which ions strike, detecting and displaying the striking ion current.

CONSTITUTION: In an ion source 4, a filament 5, a grid 6, and an ion collector 7 compose a box-like configuration and microchannel plate 8 composes the front wall. A collecting electrode 11 is an collecting electrode of the plate 8 and a through hole is formed in the center of the plate 8 and the electrode 11 and it becomes an ion outlet. Ions entering the plate 8 are converted into electrons and amplified and enter the electrode 11 and thus current in propertion to the quantity of the produced ions is generated. The current is measured by an ammeter 13 and by comparing and correcting the relation of the read value and the vacuum degree of the source 4 previously, the vacuum degree of the source 4 can be known by the ammeter 13. Consequently, without using a vacuum gauge separately, the vacuum degree of an ion source can be measured highly sensitively and highly precisely and the apparatus can be simplified.