To provide a bearing device and the like bearing a main shaft with high static rigidity, suppressing the displacement of the main shaft in the radial direction and the like, thereby enabling highly precise assembling.

Radial hydrostatic gas bearings 22, 32 bear large diameter members 11, 13 of the main shaft 1, allowing each of these hydrostatic gas bearings to have a bearing area larger than the bearing area of each of the hydrostatic gas bearings bearing a small diameter member 12, which enhances the static rigidity of the main shaft. Since these two radial hydrostatic gas bearings are disposed on the respective two large diameter members provided on the both sides of the small diameter member 12, the distance between these radial hydrostatic gas bearings is longer than the distance between the radial hydrostatic gas bearings bearing the small diameter member. Therefore, the bearing capacity of the main shaft borne by the radial hydrostatic gas bearings bearing the large diameter members is larger than the bearing capacity of the main shaft borne by the radial hydrostatic gas bearings bearing the small diameter member. This reduces the displacement of the main shaft caused by the moment load of the main shaft in the radial direction which generates when a rotation tool or the like is installed on the front side of the main shaft.