In forming a donut-like headrest under a foaming process, a sealing garnish element is provided, which includes a garnish portion and an edge abutment portion. Prior to a foaming step, the sealing garnish element is brought to a donut-like trim cover assembly and inserted fit in an annular opening formed in the annular inward wall region of the trim cover assembly, such that the edge portions of the annular opening are abutted against the edge abutment portion of sealing garnish element, while the garnish portion is attached on and along the annular inward wall region of trim cover assembly. Hence, during the foaming step, those edge abutment and garnish portions of sealing garnish element effectively prevents a liquid foaming base material injected in the trim cover assembly, against leakage from the annular opening.