An object of the present invention is to provide the X-ray tube which improves the workability of the baking for obtaining the ultra-high vacuum of the X-ray tube having a field emission type electron gun and have a stable performance. The X-ray tube comprises a field emission type electron gun chamber, an electron beam aperture, an X-ray target and a vacuum pump, in one body with a vacuum sealing structure (vacuum tube section). The vacuum tube section is attachable and detachable to the electromagnetic lens section in the X-ray tube, thereby it is possible to perform the baking by removing only the vacuum tube section. The fitting portions for positioning are provided at the vacuum tube section and the electromagnetic lens section, and therefore it is a constitution to easily perform an optical axis alignment at a mounting time after the baking.