To capture a nearby occupant by opening a vent hole in the small capacity expansion at an initial stage of the inflation and deployment of the airbag, and to capture the separately-placed occupant by allowing the vent hole to open in this state in the process of the full inflation and deployment in which the vent hole is blocked to enable a quick full inflation and deployment.

At the initial stage of inflation and deployment, a first surface forming base fabric 11 and a second surface forming base fabric 12 are joined at a joint part 13 in which each can be separated from each other at the outer peripheral side under a predetermined inflation gas pressure, and the vent hole 5 at the second surface forming base fabric 12 side is opened without being blocked by a vent hole cover 6 having a hauling part 6b. In the initial stage of inflation and deployment, the airbag 1 undergoes inflation, so that the joint part 13 is separated to form in a large capacity expansion shape. In the process of progress, the vent hole 5 is blocked by a pulling action due to elongation of the vent hole cover 6 to be in a full inflation and deployment state early, and the vent hole 5 is opened by relaxing the vent hole cover 6 when the occupant 10 is brought into contact with the airbag.