An airbag includes: an outer member configured to form an outer peripheral wall upon completing inflation, which includes a driver side portion configured to receive a driver, and a steering-wheel side portion supported by a steering wheel; and an inner member provided to expand inside the outer member. The inner member has a supply port for supplying inflation gas to the outer member and covers the inflow opening of the outer member to be arranged on an upstream side of flow of the inflation gas flowing from the inflow opening into the airbag such that a higher pressure can be maintained in the inner member than the outer member. The inner member includes a front support inflating portion configured to support a portion of the driver side portion of the outer member at least at a front side upon completing inflation of the airbag.