An airbag device is disposed beneath the lateral surface of a driver's seat backrest facing a front passenger seat, and beneath the lateral surface of a front passenger seat backrest facing a driver's seat. The air bag device has first and second airbags that are inflatable and deployable upon a collision of a vehicle equipped with airbag device. The inflated and deployed first airbag has a depressed portion on its outer side in a vehicle width direction near the shoulder of an occupant, and the second airbag inflates and deploys so as to fill the depressed portion. Upon the collision of the vehicle, when the body size of the occupant is greater than or equal to a predetermined size, only the first airbag inflates and deploys, whereas when the body size is smaller than the predetermined size, the first and second airbags inflate and deploy.