An airbag device provided with an airbag and inflator integrally covering the head and side sections of an occupant seated in a vehicle seat. The airbag, includes, during expansion and deployment, a head protecting chamber that covers the head of the occupant and side protecting chambers that cover the side sections of the occupant, and is stowed in a rolled state, for example, from a first side of the backrest of the vehicle seat to the second side over the upper portion. An additional folding part is provided on the outer periphery of the airbag extending from the attachment end side to the backrest so as to cover at least a part of the outer periphery of the airbag on the side opposite to the seated occupant.