Abstract: A vehicle seat includes a seat cushion, a seat back having a side support, a frame sidewall in the side support, and a side airbag device. The side airbag device has an airbag in the side support, and an inflator for supplying a gas to the airbag. The airbag has a first chamber to be deployed toward a vehicle front side, a second chamber, and an inner vent hole at a partition between the first and second chambers. The second chamber starts to be deployed prior to the deployment of the first chamber. The gas flows from the second chamber to the first chamber via the inner vent hole. When the second chamber is deployed, at least one of an upper region or a lower region of the second chamber protrudes toward the vehicle front side than a middle region of the second chamber.