[Subject] To provide an airbag and an airbag apparatus which are able to absorb an impact of an occupant even when a gas inflator is downsized and to fulfill demands such as downsizing, cost reduction, weight reduction, and the like of the airbag apparatus.[Solving Means] A airbag apparatus in the present invention includes an inflator (gas generator) 1 configured to generate gas under predetermined conditions, an airbag 2 being connected to the inflator 1 and configured to be inflated and deployed, and a retainer (not shown) configured to store the airbag 2, the airbag 2 includes a vent hole 3 formed on an outer shell of the airbag 2 and opening and closing means 4 configured to open and close the vent hole 3, the opening and closing means 4 is configured to maintain the vent hole 3 in a substantially closed state until inflation and deployment of the airbag 2 are completed and bring the vent hole 3 into a fully-opened state before a flattening amount of the airbag 2 reaches a predetermined reference value.