To provide an X-ray transmission window which is low in X-ray attenuation by providing a nontoxic ductile transmission window with vacuum sealing capacity capable of withstanding the working environment of an X-ray tube vacuum vessel.

A material sheet of a transmission window of aluminum or an aluminum alloy is subjected to explosive welding to the sheet of a transfer layer 30 made of stainless steel, in a single explosion process, and the laminated panel is machined to remove the transfer layer 30 from the center part. In the same way, window material is removed from a panel edge to expose a welding flange 30A to form a window assembly 28 that can be formed in required curvature. Finally, the welding flange 30A is welded to a vacuum vessel 18 made of stainless steel, in an ordinary way to form a joint 26. The vacuum vessel 18 is sealed with the combination of a vacuum seal joint 31 and the joint 26. The vacuum seal joint 31 is formed between the transmission window 24 and the transfer layer 30 through explosive welding, and the joint 26 is formed between the transfer layer 30 and the vacuum vessel 18 through ordinary welding.