Provided is a vehicle seat assembly which can prevent a submarine phenomenon in a reliable manner, and which is simple in structure so as to allow a high level of flexibility in layout. A slip preventing member for preventing the vehicle occupant from slipping forward under the seat belt in case of an impact is supported by a fixed part of a seat so as to be pivotable between a retracted position and a raised position, and an actuator is interposed between a fixed part of said seat and a part of said slip preventing member. To retain the slip preventing member at its raised position even after the power of the power unit is all used up, the assembly further includes a lock mechanism for retaining said slip preventing member at said raised position.