A structure of an auxiliary seat disposed in a second row in a vehicle is provided. The auxiliary seat storage structure includes an auxiliary seat that is disposed between a pair of exterior seats and has a seat back rotatably coupled to a seat cushion by a recliner and a rail that is installed in a vehicle width direction. The auxiliary seat is configured to slide into an internal cavity at a lower end of the exterior seat. The auxiliary seat slides along the rail when the auxiliary seat is folded and simultaneously adjusted downward and is stored in a lower cavity of the exterior seat. A passageway cavity is maximized, the number of operating steps are minimized and the auxiliary seat are stored regardless of whether an occupant is seated on the exterior seat.