The purpose of this invention is to easily conduct operation to fix a fastening elastic string in order to attach a seat back cover to a seat back main body.The means to achieve the purpose is to form a clip 201 comprising a holding part 7 provided with a groove 9 to hold a fastening elastic string 104 and a locking part 8 provided with first and second locking bodies 15, 16 in order to be locked to a fixing hole 5 of a lower panel 103, then to insert the fastening elastic string 104 to the groove 9 of the clip 201 while pulling thereof, and then to set a protrusion 3 of the fastening elastic string 104 to stopper 11b of the groove 9 with elasticity in the shrinking direction.