An airbag device installed in a vehicle seat having a seat cushion forming the seat surface and a seatback forming the backrest, and an inflator that generates expansion gas and an airbag that is deployed by the expansion gas released from said inflator. The airbag includes a pair of left and right side chambers that deploy forward from the side of the seatback and primarily protect the occupant's waist to shoulders, and an upper chamber that is continuous with the side chambers, deploys forward above the side chambers, and mainly protects the occupant's neck and head. On the left and right sides of the seatback, the location on the side where the seatbelt crosses near the shoulder is the first side, and the side opposite to said first side is the second side. A recess is formed on the first side of the airbag where the seatbelt fits.