An engagement protrusion is formed in square in cross-section integrally on an outer surface of a main body of a retainer, and is provided on a position opposed to a space for deformation so as to extend from an opposite axial end to short of an axial center of the main body of the retainer. An engagement recess is formed generally identical in cross-section to the engagement protrusion on an inner peripheral surface of a retainer holding portion so as to extend from an opposite axial end of the retainer holding portion to one axial end of a raised portion. The retainer is fitted in the retainer holding portion so that the engagement protrusion is inserted in the engagement recess.