To eliminate adverse effect of a photo-electron by providing a repeller electrode for reversing the direction of drawn-out ions, setting a collector elec trode on the grid side and the opposed side of an extraction electrode on the counter position thereto and collecting ions.

Electrons generated on a filament 2 are attracted on a grid 1 at higher potential, and reciprocate in and out of the grid 1 along a spiral orbit. The electrons collide with gas molecular of a space therebetween, it is ionized, ions fly downward from the opening of an extraction electrode 4 of lower potential than the grid 1. The ions reverse the orbit on the repeller electrode 5 and collide with a collector electrode 3. The number of the ions, namely, ion current is measured and pressure of a vacuum vessel is found. An X-ray generated on the grid 1 is shielded by the electrode 4 and does not irradiate the electrode 3, and photo-electron current therefor is not allowed to flow. Photo-electrons generated on the electrode 5 by the X-ray through the openings of the electrodes 3, 4 are returned to the electrode 5, reach the electrode 3, and do not adversely affect.