Disclosed herein is an automatic seat belt system. A guide rail is mounted on a vehicle body, and a driving tape is disposed along the length of the guide rail. A drive means is energized to move a slider to either one of two positions where a webbing restrains an occupant or releases the occupant therefrom. One end of the webbing used for restraining the occupant is supported to the slider which is maintained in engagement with each of the openings defined in the tape. In addition, the slider is movable in the longitudinal direction of the guide rail, and disposed movably with respect to the guide rail. A stopper is maintained in engagement with the slider in the webbing-restraining position so as to prevent the slider from being moved in the direction to which the webbing is pulled. When the load in the upward direction of the vehicle is applied to the slider so as to move the slider upward, an engagement portion of the slider is brought into engagement with an end of the stopper in the lower direction of the vehicle so as to prevent the slider from being moved along the guide rail.