A side airbag apparatus , comprising: an airbag cushion which is bag shaped, wound, or folded in a storage form; a predetermined holding plate with the airbag cushion attached thereto; and an inflator which is embedded in the airbag cushion and attached to a side frame along the side of the seat back of a vehicle seat together with this airbag cushion and the holding plate ; wherein the holding plate includes: a base part formed between the airbag cushion and the side frame ; and a side wall part which extends in the front-back direction of the vehicle in front of the side frame in the front-back direction of the vehicle and supports the expanded and deployed airbag cushion from the side; and the airbag cushion is divided into two chambers , that is , a basic pre-push chamber on the passenger side and a main chamber opposite the passenger, and the holding plate further includes a rear wall part which is disposed in front of the side frame , extends from the base part to the rear end of the side wall part , and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part is in surface contact with the side opposite the passenger of the expanded and deployed main chamber in the front-back direction so as to support the main chamber from the side. a predetermined holding plate with the airbag cushion attached thereto; and an inflator which is embedded in the airbag cushion and attached to a side frame along the side of the seat back of a vehicle seat together with this airbag cushion and the holding plate ; wherein the holding plate includes: a base part formed between the airbag cushion and the side frame ; and a side wall part which extends in the front-back direction of the vehicle in front of the side frame in the front-back direction of the vehicle and supports the expanded and deployed airbag cushion from the side; and the airbag cushion is divided into two chambers , that is , a basic pre-push chamber on the passenger side and a main chamber opposite the passenger, and the holding plate further includes a rear wall part which is disposed in front of the side frame , extends from the base part to the rear end of the side wall part , and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part is in surface contact with the side opposite the passenger of the expanded and deployed main chamber in the front-back direction so as to support the main chamber from the side. a base part formed between the airbag cushion and the side frame ; and a side wall part which extends in the front-back direction of the vehicle in front of the side frame in the front-back direction of the vehicle and supports the expanded and deployed airbag cushion from the side; and the airbag cushion is divided into two chambers , that is , a basic pre-push chamber on the passenger side and a main chamber opposite the passenger, and the holding plate further includes a rear wall part which is disposed in front of the side frame , extends from the base part to the rear end of the side wall part , and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part is in surface contact with the side opposite the passenger of the expanded and deployed main chamber in the front-back direction so as to support the main chamber from the side. <EOS>