An occupant protection device having a vehicle seat having a seat cushion forming a seat surface and a seat back forming a back rest, and a side air bag device accommodated in the vehicle seat, wherein the seat back has a side support part bulging in a vehicle travelling direction (vehicle forward) at a side part (end part) thereof in a vehicle width direction, a seat frame forming a framework of the seat is provided inside the vehicle seat, the side air bag device has an air bag inflating and deploying to restrain an occupant and an inflator supplying inflating gas to the air bag, a side frame part constituting a part of the seat frame is disposed inside the side support part, the side air bag device is accommodated on an outer side of the side frame part in the vehicle width direction, the air bag has a main chamber deploying forward from the side support part and a sub-chamber deploying forward from between the main chamber and the side support part, and the sub-chamber deploys further toward a forward side than a vehicle forward edge of the side frame thereby making at least a front part of the side support part protrude and deform toward an occupant side, and the side support part has a starting-point region serving as a starting point when the side support part is bent to the occupant side by inflation of the sub-chamber, characterized in, that the starting point region is formed inside the side support part, and the starting-point region is one of or a combination of a cut, a recessed part, and a thinned region. the seat back has a side support part bulging in a vehicle travelling direction (vehicle forward) at a side part (end part) thereof in a vehicle width direction, a seat frame forming a framework of the seat is provided inside the vehicle seat, the side air bag device has an air bag inflating and deploying to restrain an occupant and an inflator supplying inflating gas to the air bag, a side frame part constituting a part of the seat frame is disposed inside the side support part, the side air bag device is accommodated on an outer side of the side frame part in the vehicle width direction, the air bag has a main chamber deploying forward from the side support part and a sub-chamber deploying forward from between the main chamber and the side support part, and the sub-chamber deploys further toward a forward side than a vehicle forward edge of the side frame thereby making at least a front part of the side support part protrude and deform toward an occupant side, and the side support part has a starting-point region serving as a starting point when the side support part is bent to the occupant side by inflation of the sub-chamber, characterized in, that the starting point region is formed inside the side support part, and the starting-point region is one of or a combination of a cut, a recessed part, and a thinned region. the side support part has a starting-point region serving as a starting point when the side support part is bent to the occupant side by inflation of the sub-chamber, characterized in, that the starting point region is formed inside the side support part, and the starting-point region is one of or a combination of a cut, a recessed part, and a thinned region.