A patch element 20 of a doughnut-like disc shape is fixedly provided, by sewing or adhesive, to a reverse surface of a seat-back-side area 12a defined in a trim cover assembly section covering a top region 12 of a headrest HR, in such a fashion that the patch element 20 circumscribes an entire edge of a headrest stay passage hole 14. Additionally, the patch element 20 is formed from a material capable of being impregnated with a liquid foaming agent injected in the trim cover assembly 2. Such patch element 20 has been impregnated with the liquid foaming agent S injected in the trim cover assembly 2 during a foaming process to form the headrest HR. Consequently, a density of the patch element 20 itself becomes extremely high, and therefore a strength of the patch element 20 can be increased to a great degree.