An airbag includes a vent and a gas discharge limitation member to limit gas discharged from the vent. The limitation member includes at least one invertible portion on a surface on a side opposite the occupant-facing surface of the airbag and capable everting toward the outside of the airbag. The limitation member includes an interlocking member for interlocking movement of the occupant-facing surface in an occupant direction and an opposite direction and for interlocking movement of the invertible portion inward of the airbag and outward of the airbag when expanded. The limitation member includes a lid covering the vent and being pulled by the invertible portion when the invertible portion everts towards the outside of the airbag. The lid closes the vent or opens the vent by a small amount until the lid member is pulled by the invertible portion and opens the vent by at least a large amount.