To provide a bearing device which can maintain rigidity of bearing and improve rotation accuracy of the bearing used in inner ring rotation, while surely preventing creeping.

The bearing device includes a rolling bearing 10 in which an inner ring is out-fitted to a swivel base 32 which is a shaft, an outer ring is fitted into a stage 31 which is a housing, and a pre-load is applied after assembly into the swivel base 32 and the stage 31, with a plurality of balls arranged between the inner and the outer rings. Between the outer peripheral surface of the swivel base 32 and the inner peripheral surface of the inner ring, a loose engagement is established under such condition as a pre-load is not applied yet to the rolling bearing 10, but a tight engagement is established under such condition as the pre-load is applied to the rolling bearing 10. Here, φd_m<(φD+φd)/2 is satisfied, where φD is the outer diameter of the outer ring of the bearing, φd is inner diameter of the inner ring, and φd_m is a diameter of pitch circle of the balls. Further, under such condition as the pre-load is applied to the rolling bearing 10, the contraction amount of the inner diameter of inner ring is set to be larger than the expansion amount of the outer diameter of the outer ring.