An airbag and an airbag device are provided in which an openable-closable vent hole opens or significantly opens even if an object, such as a child seat, comes into contact with a lower portion of the airbag in an inflated state. A covering member (20) covers an openable-closable vent hole (11) from the outside of an airbag (10). An upper tether (30) routed through an upper constantly open vent hole (12) and in the inside of the airbag (10) connects the covering member and a near-passenger face (10f) of the airbag (10). A lower tether (31) routed through a lower constantly open vent hole (13) and in the inside of the airbag (10) connects the covering member and a lower portion (10b) of the airbag (10). When a passenger comes into contact with the near-passenger face (10f) of the airbag (10), the upper tether (30) loosens and the covering member (20) is turned up, whereby the openable-closable vent hole (11) opens. When a child seat (6) comes into contact with the lower portion (10b) of the airbag (10), the lower tether (31) loosens and the covering member (20) is turned up, whereby the openable-closable vent hole (11) opens.