Airbag apparatus

In order to provide an airbag apparatus (10) in which a vent hole is opened without failure when the inner pressure of an airbag has reached a predetermined level without using a vent-hole-valve drive unit, an inner bag (14) and an outer bag (16) are placed in a retainer (12) in folded conditions and the retainer (12) is closed by a module cover (20). The retainer (12) has a vent hole (22). The vent hole (22) is closed by a lid (24). The inner bag (14) has a capacity from about 1.5 times as high as that of the retainer (12) to about half of the capacity of the outer bag (16). The gas from the inflator (18) is let in the inner bag (14). Upon inflation of the inner bag (14), the module cover (20) is pushed open by the inflation pressure of the inner bag (14). The gas in the inner bag (14) is discharged into the outer bag (16) to inflate the outer bag (16). When the inner pressure of the outer bag (16) has reached a predetermined level, the lid (24) opens to open the vent hole (22).