Construction is achieved by which it is possible definitely prevent the occurrence of creep between a pulley 2b and an outer ring 7f. Trapezoidal shaped concave sections are formed such that the angle of intersection θ between the inside surfaces 23 on both sides of each concave section 17a of knurling 19a of the outer ring 7f is within the range 45°≦θ≦120°. Moreover, the depth h in the radial direction of the concave sections 17a and the diameter D of the circumscribed circle of the tip end surfaces 22 of the convex sections 18a are regulated so as to satisfy the relationship 0.004D≦h≦0.015D, and the length L in the circumferential direction of the bottom surface 21 of the concave sections 17a is regulated within the range 0.01D≦L≦0.03D.