An airbag and an airbag device are provided that can effectively prevent a rise of the internal pressure of the airbag when the airbag is hindered from being normally inflated/deployed while gas is prevented from being wasted. An airbag includes a pair of side panels that form a bag by being sewn together, an insertion hole formed in the side panel, and a valve disposed on an inner side of the side panel so as to stop up the insertion hole and having two end portions connected to the side panel. While the airbag is housed in the structure, the valve is drawn to an outer side of the side panel through the insertion hole and disposed so as to stop up the insertion hole and, while the airbag is inflated/deployed, the valve is drawn to the inner side of the side panel.