A cylindrical X-ray tube having an outer insulating layer, a cathode electrode and an anode electrode disposed at both ends of the outer insulating layer, a gate electrode disposed between the cathode and anode electrodes, an emitter, and a target, comprises an inner insulating layer which is disposed between the cathode electrode and the outer insulating layer, is formed to extend downward in a coaxial direction with the outer insulating layer, and is pre-adjusted in order to secure an insulating distance between the cathode electrode and the gate electrode. Thus, by providing a separate internal insulating layer extending coaxially with the external insulating layer between the cathode electrode and the external insulating layer, the insulating distance between the cathode electrode and the gate electrode, the insulating distance between the cathode electrode and the anode electrode may be easily adjusted, so that a desired insulating capability can be secured.