To provide further sufficient relaxation and nap in a car cabin.

Head restraints 21 are removed, and a front seat backs 16 and 17 and a rear seat back 20 are laid down rearward for forming right and left front seats 12 and 13 and a rear seat 18 into a full flat seat. The head restraints 21 are detachably assembled to the inner sides of the seat cushion 15 of the front seat 13, one of the right and left front seat, and the front seat back 17 so that the gap between the right and left front seats is blocked by the face of the full flat seat.