To apply a pre-load on rotary bearings for supporting a target and an anode of an X-ray tube so as to make them rotate.

This device has a housing 32 coupled to a frame 18 so as to fix it, and the housing is equipped with a cylindrical inner surface 34, so that it is placed so as to accept a rotary bearing 12. The device has a spring 36 to apply a force onto a bearing race in the axial direction, and selectively applies a pre-load onto a bearing ball 26 of the bearing or other rotating elements. In addition, the device is placed between the bearing race 30 and the cylindrical inner surface 34, and has a linear bearing 48 to enable moving the bearing race in the axial direction to the housing in response to the force applied in the axial direction. The linear bearing is composed of a selective number of linear bearing rows closely spaced each other, and each linear bearing row is composed of a large number of linear bearing balls disposed along the rotary bearing race as a circular arrangement.