To obtain a structure in which an outer ring 3b having insulating layers 7b on the outer circumferential surface and on both end surfaces in the axial direction is clamped from both sides in the axial direction, and which can obtain a sufficiently large creep preventing effect even in the case that the magnitude of a tightening force is limited.

In a portion of insulating layers 7b existing on both end surfaces in the axial direction of an outer ring 3b, recessed grooves 12, 12 extending in the circumferential direction are formed. Then, by reducing an area where both the end surfaces come into contact with a stepped surface 14 and a pressing surface 16 clamping the outer ring 3b from both sides in the axial direction by an amount corresponding to areas of openings of both the recessed grooves 12, 12, surface pressure at the contact portion is raised. By this, without the need to particularly increase a tightening force, the contact portion between the surfaces brings about a mechanically engaged state that minute protruding parts bite into the other surface, thereby magnifying a creep preventing effect.