A side airbag apparatus (100,200,300), comprising: an airbag cushion (108,202) which is bag shaped, wound, or folded in a storage form; a predetermined holding plate (118,134,142,204,302) with the airbag cushion (108,202) attached thereto; and an inflator (110,206) which is embedded in the airbag cushion (108,202) and attached to a side frame along the side of the seat back of a vehicle seat together with this airbag cushion (108,202) and the holding plate (118,134,142,204,302); wherein the holding plate (118,134,142,204,302) includes: a base part (120,208,306) formed between the airbag cushion (108,202) and the side frame; and a side wall part (122,210,304) 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 (108,202) from the side; and the airbag cushion (108,202) 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 (118,134,142,204,302) further includes a rear wall part (124,212) which is disposed in front of the side frame, extends from the base part (120,208,306) to the rear end of the side wall part (122,210,304), and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part (122,210,304) 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 (118,134,142,204,302) with the airbag cushion (108,202) attached thereto; and an inflator (110,206) which is embedded in the airbag cushion (108,202) and attached to a side frame along the side of the seat back of a vehicle seat together with this airbag cushion (108,202) and the holding plate (118,134,142,204,302); wherein the holding plate (118,134,142,204,302) includes: a base part (120,208,306) formed between the airbag cushion (108,202) and the side frame; and a side wall part (122,210,304) 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 (108,202) from the side; and the airbag cushion (108,202) 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 (118,134,142,204,302) further includes a rear wall part (124,212) which is disposed in front of the side frame, extends from the base part (120,208,306) to the rear end of the side wall part (122,210,304), and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part (122,210,304) 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 (120,208,306) formed between the airbag cushion (108,202) and the side frame; and a side wall part (122,210,304) 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 (108,202) from the side; and the airbag cushion (108,202) 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 (118,134,142,204,302) further includes a rear wall part (124,212) which is disposed in front of the side frame, extends from the base part (120,208,306) to the rear end of the side wall part (122,210,304), and supports the expanded and deployed main chamber from the rear side of the vehicle; and the side wall part (122,210,304) 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.