PURPOSE: To suppress the temperature rise of a discharge window, and prevent any deformation and breakage of the discharge window by reflecting radiation heat sent from a target by providing a heat reflecting body on the inner wall of the discharge window so that the said heat reflecting body faces the target.

CONSTITUTION: The inner wall of a discharge window 4 facing a target 2 is provided with a heat reflecting body 10 which is formed from Ni or Cu by plating, vapordeposition or the like, so that any temperature rise of the window 4 is prevented by reflecting radiation heat sent from the target 2. It is desirable that the plating or vapordeposition is performed on the entire inner wall of the window 4 except the area of the X-ray radiation. Here, it is revealed that the temperature of the window 4 of this invention is lower than that of the conventional one by 8°C∼10°C. In the figure, the symbol 11 represents a discharge- window attachment screw. By using such a discharge window as specified in this invention, any deformation of the discharge window can be prevented without reducing the allowed temperature of an X-ray tube device.