A side air bag device restraining an occupant at the early stage of the deployment. A seat back has a side support part bulging in a vehicle forward direction at a side part in a vehicle width direction. A seat frame of the seat is provided inside the seat. The side air bag device has an air bag deploying to restrain an occupant and an inflator supplying inflating gas. A side frame part of the seat frame is inside the side support part, and the side air bag device is on an outer side of the side frame. The air bag has a main chamber deploying forward and to an outer side of the side support part and a sub-chamber deploying inside the side support part. The sub-chamber deploys further toward a forward side than a vehicle forward edge of the side frame, at least a front part of the side support part protrudes and deforms toward an occupant side.