To provide an airbag and an airbag device that can keep the internal pressure of the airbag in the initial stage of inflation and deployment high. An airbag includes a first chamber and a second chamber. The first chamber includes a nozzle forming a flow path capable of communicating with the second chamber, and a tether connecting the inner surface of the nozzle and the inner surface of the first chamber. During the inflation and deployment of the first chamber, the supply of gas to the second chamber is prevented by retracting the nozzle into the first chamber. After the first chamber is expelled into the cabin and when a predetermined external pressure is loaded on the first chamber, the nozzle is expelled into the second chamber so as to allow gas to be supplied to the second chamber.