An X-ray tube is disclosed. The X-ray tube includes a substrate, a box-shaped case attached to the substrate and being in a high-vacuum state, an X-ray target arranged in the opening of the first substrate in the inside of the case, and a cathode arranged in the case and supplying an electron to the X-ray target. The substrate includes first and second substrates made of 426 alloy and respectively having an opening of honeycomb structure, and an X-ray transmissive window sandwiched between the first and second substrates which is made of a titanium foil and close the opening. The X-ray transmissive window is reinforced by a honeycomb structure of the substrate from both surfaces. Thus, the substrate and the X-ray transmissive window are not deformed, and strength of the package is improved.