To provide an airbag device capable of reducing total damage from a lower limb to an upper body in a collision predictive action, and reducing damage to an occupant even in a restricted posture close to an airbag when the airbag is deployed.

A bag module fitted to a seat belt 1A with a folded airbag 11 stored therein is loaded on a belly of an occupant D while the occupant D is seated. When a collision is predicated or a collision is detected, gas is fed into the airbag 11 in the bag module, and the airbag is inflated and deployed so as to cover the belly of the occupant D by a part 11B of the airbag 11 and to cover a front side in a range from the belly to a lower limb by a part 11A of the airbag 11. Damage from the lower limb to the upper body of the occupant D can be reduced as a whole, and a shock load on the occupant can be reduced even if an airbag at a driver's seat or the like is deployed.