A sliding door attached to a vehicle is supported on the body of the vehicle by way of a sliding door supporting apparatus, which comprises a center guide mechanism, an upper guide mechanism, and a lower guide mechanism. The center guide mechanism is provided with a holdback mechanism for restraining the sliding door from shifting its position in the transverse direction when the door is moved to a fully open position. The holdback mechanism includes a vertical wall formed on a center rail, inside and outside holdback rollers of which the positions in the transverse direction are restrained by the vertical wall. The upper and lower guide mechanisms are provided, respectively, with upper and lower check mechanisms for holding the sliding door in the fully open position. The center and lower guide mechanisms are each provided with a roller for supporting vertical load of the sliding door.