The occurrence of wear during use in areas of contact between residual protrusions 22a and opposing end surfaces due to residual protrusions 22a that remain on end surfaces in the circumferential direction of retainer elements 15b after shearing of small cross-sectional area sections is prevented. Convex sections 23, the tip ends thereof being flat, are provided on one end surface in the circumferential direction of retainer elements 15 so as to protrude in the circumferential direction. The arrangement, shape and dimensions of the small cross-sectional area sections 17a and the convex sections 23 are regulated so that when the retainer elements 15b are aligned with each other in the axial direction, and the tip-end sections of the convex sections 23 come in contact with the other end surfaces in the circumferential direction of the retainer elements 15b, spaces 24 in the circumferential direction exist between the tip-end sections of the residual protrusions 22a and the end surfaces in the circumferential direction of the retainer elements 15b.