An airbag includes a bag portion including two sheet members which are attached to each other. The bag portion provided with the sheet members which are attached to each other includes a peripheral rim portion having a plurality of bent side portions, the bent side portions which each is bent by filling with air inside the bag portion, the bent side portions, the bent side portions which each is formed with a groove-shaped constriction at a surface of the bag portion which is configured to inflate by filling with air.