An X-ray tube comprises: an envelope which has a cathode at one end and an anode at another end of a barrel of a tubular insulating tube and which has a sealed interior; an electron gun which is arranged inside the envelope and has a shape that protrudes from the cathode; and a target which is electrically connected to the anode and generates X-rays when being irradiated with electrons emitted from the electron gun. With reference to an end position that is a projection of a position of an end on the anode side of the electron gun onto an inner wall of the insulating tube, a mean wall thickness of the barrel is greater on the cathode side than on the anode side.