An airbag device installed in a vehicle seat having a seat cushion forming a seat surface and a seatback forming a backrest, being provided with an inflator stowed in one side portion of the seatback that generates inflation gas, and an airbag stowed in the seatback in a rolled or folded state that deploys from one side portion by the inflator releasing the inflation gas. The airbag deploys toward the front of the seatback and includes an upper chamber region for protecting at least around the head and neck of the occupant and a lower chamber region for protecting mainly from the waist to the shoulders of the occupant.