PURPOSE: To achieve long-life rotation of low vibration by installing a pair of bearings to enable the outer ring race to smoothly move due to a preliminary pressure in the thrust direction which is applied to the bearings by a coil spring.

CONSTITUTION: One end of an outer ring race 17 touches the lower end of a coil spring 8 for preliminarily applying a pressure through an annular spacer 27, so that a given preliminary pressure is constantly applied in the thrust direction. An auxiliary ball 25 located on the outer surface of the outer ring race 17 touches the inner flat surface of an auxiliary outer ring race 26, so that the auxiliary ball 25 can freely move in the thrust direction. It is possible to minimize the frictional force between the outer ring races 17 and 26 because they touch each other through the auxiliary ball 25. Consequently, a bearing 14 reliably moves in the thrust direction due to the temperature difference and the thermal expansion difference of the parts of the rotor, thereby increasing the life of a rotary-anode-type X-ray tube.