Various airbag systems such as driver-seat airbag systems have been used to protect occupants in case of an emergency such as a car crash. Driver-seat airbag systems include an airbag formed by sewing the peripheries of a front panel adjacent to the occupant and an opposite rear panel together. The rear panel has an inflator-engaging opening for receiving the end of an inflator in the center. The periphery of the opening is fixed to a retainer with bolts, pins, or rivets. The rear panel also has a vent hole for releasing gas in the airbag to absorb the impact when a driver-seat occupant strikes against the airbag.
It is described in Japanese Unexamined Patent Application Publication No. 1-311930 to provide an airbag having an inner panel (referred to as an inner airbag in this reference) between the inflator opening of the rear panel and the front panel to partition the interior of the airbag into a central first chamber and a peripheral second chamber. The inner airbag has a communication hole for communicating the first chamber and the second chamber with each other. Upon activation of the inflator, the first chamber inflates first and then the second chamber inflates.