PURPOSE: To enable the vibration in both radial and axial directions to be restrained by a rolling bearing, by bringing the outer circumferential surface of a small diameter part and the stepped surface connected to the small diameter part to face the inner circumferential surface of the inner ring of the rolling bearing and the side surface thereunder, respectively.

CONSTITUTION: In a ball bearing 37, the inner circumferential body of an inner ring 39 is faced to the outer circumferential surface of the small diameter part 25 of a turning shaft 20 so as to keep an appropriate radial clearance between them, while the under side-surface of the inner ring 39 is faced to the stepped surface 26 in the vicinity of the rotor fitting part 24 of the turning shaft 20 so as to keep an appropriate axial clearance between them. Accordingly, when an abnormal vibration is caused, the outer circumferential surface of the small diameter part 25 of the turning shaft 20 is brought into contact with the inner circumferential surface of the inner ring 39 of the ball bearing 37, and the stepped surface 26 with the under side-surface of the inner ring 39, so that the abnormal vibration can be restrained by the ball bearing 37.