An airbag is constructed such that a gas-discharging amount from a venthole becomes largest when an occupant contacts a center portion, in the left and right direction, of an occupant-facing surface of the expanded airbag. In the airbag, a hanging strap couples a center portion of a front panel and a rear panel. A lid body covers an open-and-close venthole from outside the airbag, and the hanging strap is coupled by a tether. In a case that the airbag is expanded, the hanging strap and the tether are tensed, and the lid body is brought to a state to close the open-and-close venthole. A loosening amount of the hanging strap and the tether becomes largest when an occupant contacts the center portion of the front panel to retreat, and an opening amount of the open-and-close venthole also becomes largest.