An eccentric groove 10 is formed in an outer peripheral surface of an outer ring 5 and an outer peripheral surface of an eccentric ring 1a shaped like a segmentary ring is engaged with an inner peripheral surface of a housing 3 by friction. An elastic deformation part having a predetermined length dimension in the circumferential direction is formed in a part of the eccentric ring 1a. The elastic deformation part projects between a bottom face 11 of the eccentric groove 10 and the inner peripheral surface of the housing 3 in the mounting state in the housing 3. Resultantly, the rotation prevention effect of the outer ring 5 with respect to the housing 3 is improved.