PURPOSE: To lengthen service life by forming a metallic film having specified hardness and thickness over the outermost surface of, at least, one sliding surface out of those such as an outer circumferential surface acting as the sliding surface of each ring, an inner circumferential surface acting as the sliding surface of each outer ring, an inner circumferential surface acting as the sliding surface of each ball, and a surface acting as the sliding surface of each ball when the inner ring or outer ring of each bearing is slid on each ball.

CONSTITUTION: A device is furnished with a cathode 3 generating thermal electron beams 2, and with a target 4 which generates X-rays 7 when the thermal electron beams 2 are irradiated thereon. A metallic film 13 is formed on the outermost surface of, at least, one sliding surface out of those such as each outer circumferncial surface acting as the sliding surface 10a of each inner ring 10, each inner circumferential surface 11a acting as the sliding surface of each outer ring 11, and each surface acting as the sliding surface of each ball 12 when the rotating shaft 4a of the target 4 is rotated so that the inner ring or outer ring of each ball bearing 6 is slid on each ball 12. Vickers hardness of the film 13 shall be Hv50 to 200, and the thickness of the film 0.05 to 2μm. By this constitution, a X-ray generating tube can be provided, which is excellent in fatigue flaking resistance characteristics at the sliding surface of each bearing 6 of its rotating anode, and also is long in service life.