An airbag apparatus comprising an airbag module installed in a backrest of a vehicle seat is provided. The backrest has a side support on each side. Each side support has a base portion located at a position closer to a center of the backrest. The airbag module is installed in a side portion of the back rest that faces the exterior of the vehicle. The airbag module includes a gas generator that generates gas in response to impact from a side of the vehicle, and first and second airbag portions that are inflated and deployed by the gas generated by the gas generator. The airbag module is installed in the backrest in such a manner that, when the airbag is inflated and deployed, the first airbag portion pushes the base portion of the side support on the exterior side obliquely forward and further into the passenger compartment.