To enhance an occupant restraining force of an airbag, and to improve a protective function for an occupant of an airbag device by discharging gas without fail regardless of a position of the occupant, an airbag (10) is divided into a first chamber (10A) on an inflator (2) side and a second chamber (10B) on an occupant side by a partition wall (11), and the first chamber (10A) includes an opening (23) to which a discharge-state switching member (30) having vent holes to discharge the gas in the airbag (10) and being switchable between the discharge/non-discharge states is attached. The discharge-state switching member (30) is coupled to the partition wall (11) by a tether belt (40), so that the tether belt (40) applies tension to close the vent holes when the airbag (10) inflates, and when the airbag (10) receives an occupant, releases the tension depending on the position of the receipt, and opens the vent holes.