PURPOSE: To attempt the improvement of accuracy in detecting the rotating speed of a target by comparing a frequency having the appearance of a peak in the frequency component of a vibration detection signal with a frequency equivalent to a specified rotating speed, and then judging the rotating speed of the target on the basis of the comparison results.

CONSTITUTION: A glass bulb 2 which is an evacuated vacuum envelope 2 has a cathode 3 generating thermions A, a target 4 which is a rotation anode receiving the thermions A to generate X-rays B, a motor rotor 5 rotating the target 4, etc., therein. The frequency component of a vibration detection signal from a vibration detecting means 13 installed on the side of the target 4 in the envelope 2 is found for detecting a peak in this frequency component, so that a frequency having the appearance of this peak is compared with a frequency equivalent to the specified rotating speed of the target 4. Then, the rotating speed of the target 4 is judged on the basis of the comparison results. This makes it possible to very accurately detect whether or not the target 4 has attained the specified rotating speed.