PURPOSE: To reduce unnecessary X-ray emission by providing an X-ray window of small metal content and an insulating part of small X-ray path permeability, and by high vacuum-sealing the inside of an X-ray tube main unit.

CONSTITUTION: An anode 21 is provided on an internal wall of an X-ray tube main unit 20, and a hot cathode 22 is arranged on the center axis of the anode 21. An X-ray transmission window 23 is provided in one opening part of the main unit 20, and the window 23 is formed of material of small metal content. An insulating part 24 is provided in the other opening part of the main unit 20, and this insulating part 24, using a lead glass, shields an X-ray. Since the insulating part 24 is sealed with a heater wire 25, its periphery is formed of CP glass, and the inside of the main unit 20 is sealed with a high vacuum to maintain the high vacuum by using a getter 26. Here, a current is allowed to flow in the cathode 22 to heat it through the heater wire 25, and high voltage is applied between the cathode 22 and the anode 21. Then, a thermion is emitted from the cathode 22 to collide against the anode 21, and X-ray is generated. Of this X-ray, an X-ray in an opposite direction to the ordinary one is attenuated by the lead glass of the insulating part 24.