A rail unit includes a lower rail, which guides the movement of a guide member connected to a sliding door, a front bracket, which is provided on the lower rail and fixed to a vehicle body, a rear bracket, and a center bracket. The rail unit is inserted into a first insertion hole, which is located below a lower panel provided on the vehicle body at a position below a door opening and opens outward in the vehicle body in the lateral direction of the vehicle. A drive unit is inserted into a second insertion hole, which is formed in the lower panel and opens upward in the vehicle. The drive unit is layered on an upper side of a drive pulley provided in the center bracket to rotate the drive pulley.