As a conventional configuration of a seat back frame for an automobile rear seat, JP 2003-009985 A discloses a seat frame, which comprises a U-shaped main frame, a lower frame connecting lower portions of the main frame, a longitudinal pipe extending between an upper portion of the main frame and the lower frame to bridge a gap therebetween, and a lateral pipe extending between one of right and left frames constituting the main frame and the longitudinal pipe to bride a gap therebetween.  In a vehicle seat including such a seat back frame, a seat belt retractor configured to retract a seat belt for a center seat is attached to the lateral pipe through a bracket. 
However, according to the conventional configuration of the vehicle seat, an occupant seated on the vehicle seat may experience an uncomfortable contact feeling due to the presence of the lateral pipe of the seat back frame, and the occupant's seating feeling may be impaired.  Further, the seat back frame includes a large number of constituent parts, which disadvantageously leads to an increase in workload for assembly of the seat back frame and an increase in weight of the seat back frame. 
In view of the above, it would be desirable to provide a vehicle seat which can improve the occupant's seating feeling and which can be comprised of a reduced number of parts to reduce the workload on the assembly and to reduce the weight of the seat back frame.