A vehicle seat, comprising: a seat cushion forming a seating face; a seat back which forms a backrest and has a side support part swelling in the vehicle traveling direction on the vehicle width direction side; a frame side wall part which is arranged inside the side support part and extends in the vehicle traveling direction when the horizontal cross section is seen from above; and a side airbag apparatus which comprises: an airbag which is arranged inside the side support part and expanded and deployed so as to restrain a passenger; and an inflator which is provided outside the frame side wall part in the vehicle width direction so as to supply expansion gas to the airbag, wherein: the airbag comprises: a first chamber which is deployed towards the front of the side support part ; a second chamber and a gas guide , and an internal vent hole , through which the expansion gas introduced by the gas guide flows into the second chamber , is formed at the boundary part between the first chamber and the second chamber , the second chamber includes: an upper region and a lower region ; and an intermediate region disposed between the upper region and the lower region , and at least one of the upper region or the lower region is configured to be deployed so as to protrude more to the front of the vehicle than the intermediate region , characterized in that the second chamber starts being deployed inside the first chamber in the vehicle width direction prior to this first chamber ; and the gas guide is arranged in the first chamber . a frame side wall part which is arranged inside the side support part and extends in the vehicle traveling direction when the horizontal cross section is seen from above; and a side airbag apparatus which comprises: an airbag which is arranged inside the side support part and expanded and deployed so as to restrain a passenger; and an inflator which is provided outside the frame side wall part in the vehicle width direction so as to supply expansion gas to the airbag, wherein: the airbag comprises: a first chamber which is deployed towards the front of the side support part ; a second chamber and a gas guide , and an internal vent hole , through which the expansion gas introduced by the gas guide flows into the second chamber , is formed at the boundary part between the first chamber and the second chamber , the second chamber includes: an upper region and a lower region ; and an intermediate region disposed between the upper region and the lower region , and at least one of the upper region or the lower region is configured to be deployed so as to protrude more to the front of the vehicle than the intermediate region , characterized in that the second chamber starts being deployed inside the first chamber in the vehicle width direction prior to this first chamber ; and the gas guide is arranged in the first chamber . the airbag comprises: a first chamber which is deployed towards the front of the side support part ; a second chamber and a gas guide , and an internal vent hole , through which the expansion gas introduced by the gas guide flows into the second chamber , is formed at the boundary part between the first chamber and the second chamber , the second chamber includes: an upper region and a lower region ; and an intermediate region disposed between the upper region and the lower region , and at least one of the upper region or the lower region is configured to be deployed so as to protrude more to the front of the vehicle than the intermediate region , characterized in that the second chamber starts being deployed inside the first chamber in the vehicle width direction prior to this first chamber ; and the gas guide is arranged in the first chamber . <EOS>