An X-ray tube includes an electron gun, a target that generates X-rays, and a vacuum housing that accommodates the electron gun and the target. The vacuum housing has a metal portion having an X-ray emission window, and an insulation valve connected to the metal portion. The metal portion has a cylinder portion in which the X-ray emission window is provided and which surrounds a tube axis of the vacuum housing, and a tapered portion which is connected to an end portion of the cylinder portion, surrounds the tube axis, and protrudes such that a connection part between the metal portion and an insulation valve is covered. The tapered portion has a shape increased in diameter such that a separation distance between a distal end portion and the tube axis is longer than a separation distance between a base end portion and the tube axis.