To ensure simple and quick mounting of a child seat, prevent a striker from giving a foreign or rough feel to a seated person while no child seat is used, and prevent a pad material and the striker from being revealed out through cutout portions.

A striker 21 is covered with a pad material 15 and a facing material 17. The pad material 15 and facing material 17 have respective cutout portions 23 and 25. The cutout portion 25 of the facing material 17 is closed by a flap part 31 that permits it to be reopened. The flap part 31 has a resin hook 33 on the underside. The resin hook 33 comprises a platelike mounting portion 33a and a substantially J-bent hook portion 33b. The mounting portion 33a of the resin hook 33 is attached to the underside of the flap part 31. With the cutout portion 25 of the facing material 17 closed by the flap part 33, the hook portion 33b is locked to the striker 21.