Closing movement of at least one of two or more air chambers is started at an early stage, and an inflated air chamber is tightly closed and maintained in an inflated state for a long time. Inside of an airbag is partitioned into air chambers, with a partitioning cloth, and a diffuser that stores a gas generator is disposed in the air chambers, through the partitioning cloth. Gas from a gas generator is rectified by the diffuser, and is supplied from outlet parts, into the air chambers, thereby inflating and deploying the air chambers. An occupant side base cloth, a semi-perimeter portion around the lower outlet part, is connected to a back-side base cloth of the airbag, and in accordance with an inflation of the lower air chamber, a tension is imposed on the occupant side base cloth from the back-side base cloth, thereby elongating the lower air chamber. This causes the base cloths to come close to each other and gradually narrow the opening width, thereby enabling the base cloths to closely stick to each other so as to close the opening of the lower outlet part.