Abstract: An upper cross member and a lower cross member are formed integrally together to include a cross member having the shape of a box in cross section, and to the front and rear sides thereof are joined floor panels of an automobile. A trunk room or storage space is formed between right and left side frames to which are joined both ends of the cross member. A lid for opening and closing an opening located in space between a front seat and a center seat of the automobile is pivotally supported by the cross member that forms a portion of the upper wall of the trunk room. The lid is normally locked to the floor panel by a locking mechanism having a grip. The lid when opened is maintained at its open position by anchoring a hook attached to an end of a wire to an engaging portion provided at the back of the front seat.