PURPOSE: To enable the measurement of vacuum of a vacuum working chamber even during the bleeding of a gas from an ion collector, by arranging an introduction bar connected to a current introducing section outside the vacuum at both ends of the ion collector as projecting out of a glass wall.

CONSTITUTION: A first introducing bar 8 is provided at one end of an ion collector 6 while a second introducing bar 9 at the other end thereof both projecting out of a glass wall 3. Then, with the closing of a switch 11 for bleeding gas from the ion collector, the collector 6 is electrically energized with a battery 10 to heat it, which gasifies unstable molecules adsorbed on the collector 6, making them separate from the collector 6 to be discharged outside with a vacuum pump. Thus, the bleeding of the gas is performed by heating with the energization of the collector 6, in noway affecting optimum set potential difference for the measurement of vacuum of a filament 4 and a grid 5. This enables the measurement of vacuum of a vacuum working chamber 1 accurately during the bleeding of gas from the collector 6.