An airbag device has a bag housing case, an airbag housed in the bag housing case, a plate member provided within the airbag, and a bag pressure release mechanism. When the internal pressure of the airbag reaches a predetermined pressure, the bag pressure release mechanism opens the airbag to prevent an increase in the internal pressure of the airbag. The plate member covers the bag pressure release mechanism when the airbag is in a deflated state. When the airbag is inflated, the plate member is released from coverage of the bag pressure release mechanism by an inflating force.