A vehicle seat rail assembly (10) has a lower rail (12) having a guide groove (20) opening upward in the vertical direction of the vehicle body, that extends in a direction coincident with the longitudinal direction of the vehicle, and supported stationary relative to the vehicle body; an upper rail (14) attached to a vehicle seat and guided by the lower rail (12) to slide in the longitudinal direction of the vehicle; and a cover mechanism (28) that is disposed between a pair of inner walls (12D) that define the guide groove (20) of the lower rail (12) and that closes the guide groove (20). The cover mechanism (28) includes a stationary cover (30) that remains stationary relative to the lower rail (12) and a movable cover (35) that slides relative to stationary cover (30).
