PURPOSE: To prevent generation of poor withstand voltage, with an ignition-at- exhaust filament not separated in case of being broken, by preparing a cover having an opening, at the outlet of a focusing slot in which the ignition-at- exhaust filament is housed.

CONSTITUTION: While a focusing electrode 1 with a V-shaped top surface is provided with focusing slots 2 and 3, with a coiled ignition-at-exhaust filament 4 being housed in the slot 2 with a coiled operation filament 4 in the slot 3, to form a cathode body of a X-ray tube, a cover 13 having an opening 12 smaller than a filament coil in diameter is prepared to cover the outlet of the focusing slot 2. Therefore, when the anode target is heated to high temperature by electron impact for degassing, the ignition-at-exhaust filament 4 can be prevented from separating even in case of being carbonized and partly broken, and then the resultant poorness of withstand voltage can be prevented beforehand.