PURPOSE: To secure the fixation of a rotary anode even during high-speed operation or reverse-direction rotation by forming a narrowed section on the graphite- target-plate side area of the outer circumference surface of a nut for fixing a graphite target plate to a supporting shaft.

CONSTITUTION: A part of the outer circumference surface of a nut 10 on the side of a graphite target plate 11 is provided with a stepped narrowed section 12. After the graphite target plate 11 is fitted around a supporting shaft 13, the nut 10 is tightened around a threaded section 15 through a washer 14. The shrinkage of the nut 10 is prevented by squashing the thread screw located in the end 17 of the threaded section 15 after a cradle 16 consisting of separate plates or bars is inserted in the narrowed section 12. After that, the cradle 16 is removed from the narrowed section 12 of the nut 10. By the means mentioned above, any great mechanical impact against the graphite target plate 11 is not caused during the squenching. In addition, the nut 10 and the supporting shaft 13 can be tightly fixed together without causing any damage such as cracks to the graphite target plate 11.