A seat belt assembly unit includes a seat belt retractor, seat back, seat back frame, a rear wall interconnecting the side walls of a vehicle, a tubular member extending along the length of the rear wall and a belt guide disposed adjacent to the seat back and rigidly attached to the tubular member. The seat belt guide is a rod-shaped member formed substantially into a U-shape with its side portions inserted into holes on the tubular member. A shoulder belt extending from the retractor and passing through the seat belt guide, restrains occupants to the seat. The pulling force of the shoulder belt which restrains the occupant in the seat is transferred to the tubular member via the belt guide.