Abstract: This invention relates to an automatically fastening seatbelt assembly which causes an occupant of a vehicle to be automatically held to a seat in the vehicle by a restraining webbing, after the occupant is positioned on the seat in the vehicle.The seatbelt assembly includes a guide rail, a slider and adjustment device. The guide rail includes a longitudinal groove therein, and the guide rail is mounted on a vehicle body. The slider is displaceable along the longitudinal groove of the guide rail and a restraining webbing is fastened at one end thereof to the slider. The adjustment device includes a stop member and the adjustment device is slidably mounted on the guide rail. The stop member is displaceable with the adjustment device along the guide rail and the adjustment means is adapted to be selectively position and locked at one of a plurality of anchor positions. The stop member crosses a path of the slider along the guide rail.