When an attachment circular-cylinder section coupled to one axial line direction end portion of a mainbody circular-cylinder section of a grommet is inserted into an opening portion in an upper frame, engagement claws formed at an outer periphery of the attachment circular-cylinder section engage with a peripheral edge portion of the opening portion by elastically deforming, enabling the grommet to be attached to the upper frame. When a seatback spring is passed through the grommet, a retaining section formed at an inner peripheral portion of a boundary between the mainbody circular-cylinder section and the attachment circular-cylinder section retains the seatback spring. An internal-diameter of the retaining section is smaller than internal-diameters of the mainbody circular-cylinder section and the attachment circular-cylinder section, and both inner peripheral faces of the mainbody circular-cylinder section and the attachment circular-cylinder section have cone shape with a decreasing diameter on progression toward the retaining section.