[Problem] To provide a side airbag apparatus capable of efficiently improving passenger protection performance and passenger restraining force.
[Resolution means] A side airbag apparatus 100 includes: a cushion 108 which is bag shaped, wound, or folded in a storage form; a holding plate 118 with the cushion 108 attached thereto; and an inflator 110 which is embedded in the cushion 108 and attached to a side frame 112 along the side of a seat back 104 of a seat 102 together with the cushion 108 and the holding plate 118. The holding plate 118 includes: a base part 120 formed between the cushion 108 and the side frame 112; and a side wall part 122 which extends in the front-back direction of the vehicle in front of the side frame 112 in the front-back direction of the vehicle and supports the expanded and deployed cushion 108 from the side.