A lid 24 is constituted by an extending portion 41 of a first carpet 35 extending from the front of a lower portion of a seat cushion 21, a second carpet 43 integrally mounted on the extending portion 41 via a base material 42, and the first carpet 35 is made to function as a hinge 45. In addition, the lid 24 is constructed so as to be retained to a floor 15 with a surface fastener 44.