PURPOSE: To maintain high vacuum in a tube for long term with simple structure without requiring special heating operation, by fixing a zirconium board at the position on the side or bottom of a focusing electrode where the temperature will rise to specific level when preheating the filament.

CONSTITUTION: When using an X-ray tube continuously or preheating a filament, zirconium boards 7, 7' or a board coated with zirconium or a container is fixed at the position on the side or bottom of a focus electrode 3 where the temperature will rise due to the heating of the filament to 300W400°C suitable for the zirconium to perform effective gettering. When heating the zirconium to 300W400°C good gettering is provided and functions effectively even for the hydrogen. Consequently high vacuum can be maintained for long term with simple structure without requiring special heating.