An airbag and an airbag device are provided in which a posture of inflation and deployment of the airbag can be held easily and simply even if the airbag is mounted on an instrument panel having a convex portion in the space of inflation and deployment of the airbag. An airbag device includes an airbag that is folded and housed in an opening portion of an instrument panel during a normal state and is inflated and deployed on a surface of the instrument panel by gas supplied in an emergency; an inflator that supplies gas to the airbag; a retainer that holds the airbag and the inflator; and an airbag cover that covers the airbag and configures a part of the instrument panel, in which the airbag has a concave portion formed in both lower side portions when the airbag is inflated and deployed.