To provide an X-ray tube which can produce a high-definition perspective image through micronization of an X-ray tube focus in a wide range of tube voltage.

Electron emitted from a thermionic cathode 1 is drawn out by a control grid electrode 2, which electron beam is accelerated by a perforated positive electrode 3. A motor with a decelerator 25 is operated to move the perforated positive electrode 3 to a driving direction 26 through screw parts 22, 21, with which a crossover diameter of the electron beam is diminished, and the electron beam is introduced into a vacuum pipe 12. Then, a direction is adjusted by a convergence coil 4, and the beam is irradiated at a target by converging it with the convergence coil. An X-ray tube focus is micronized by moving the perforated positive electrode 3 to and fro according to the tube voltage, and a high-definition perspective image can be obtained.