A curtain air bag of a curtain airbag system includes a main chamber stored in a folded condition in a roof side portion that extends rearward from a front pillar, and a front auxiliary chamber formed integrally with the main chamber and stored in a folded condition in the roof side portion together with the main chamber. The main chamber is inflated with gas supplied thereto in the event of a side impact of a vehicle and rollover of the vehicle, to be deployed like a curtain downwardly of the vehicle. The front auxiliary chamber is inflated with gas supplied thereto at least in the event of rollover of the vehicle, to be deployed in front of the main chamber such that an upper portion of the front auxiliary chamber as seen in a side view overlaps the front pillar, and a lower portion thereof overlaps a front side door.