Hitherto, various airbag devices, such as a driver airbag device that is inflated for deployment towards a driver from a rotational center of a steering wheel at a driver's seat, and a passenger airbag that is inflated for deployment towards a passenger's seat from an instrument panel, are used for restraining the body of an occupant when, for example, an automobile collides. 
In recent years, an airbag device has already been proposed (refer to, for example, Japanese Unexamined Patent Application Publication No.  2002-37011) to overcome the following.  That is, when a serious accident that applies a large external force to an automobile occurs, for example, overturning of a vehicle body, resulting from a collision, or falling of a heavy object, such as a tree, a shock may be due to the shock generated during the serious accident. 
In the aforementioned related art, the following basic structure is discussed.  That is, an airbag, formed of a bag member in which base cloths are joined to each other, is accommodated in a backrest (or a headrest) of a seat of a vehicle, and the airbag is inflated by supplying a pressure fluid from an inflator in an emergency, to deploy the airbag towards the upper side of the head of an occupant.