An X-ray tube is provided with: a cathode and an anode sealed inside a vacuum envelope; and an ion-collecting conductor attached to the vacuum envelop so as to be in contact with an internal space of the vacuum envelope. A first current sensor measures a value of a first current flowing between the ion-collecting conductor and a node for supplying potential for attracting positive ions in the vacuum envelope. A second current sensor measures a value of a second current flowing between the anode and the cathode. A control circuit generates diagnostic information on the degree of vacuum of the X-ray tube based on a current ratio file of the first current value (Ii) measured by the first current sensor to the second current value (Ie) measured by the second current sensor.