An air bag device to be installed in a vehicle provided with a front seat and a rear seat disposed away from each other in a front-rear direction of the vehicle, and a reclining mechanism provided in the front seat to allow a seat back to pivot about a reclining pivot axis includes an air bag that deploys from a back surface of the seat back of the front seat toward the rear seat. At least an upper rear portion of the air bag in a deployed state is formed into an arc shape about the reclining pivot axis in a side view of the vehicle.