AIRBAG DEVICE

An airbag includes a main inflation portion to be deployed and inflated at an outer side of the upper body of an occupant, and an auxiliary inflation portion disposed at a position adjacent to an inner side of the main inflation portion and to be deployed and inflated at an obliquely front outer side of a head portion. In the airbag whose deployment and inflation are completed, a maximum dimension in a facing direction in which an outer main body portion and an inner main body portion of the auxiliary inflation portion face each other is smaller than a dimension in a direction orthogonal to the facing direction. The auxiliary inflation portion has the outer main body portion as a joining area, and is joined to the main inflation portion in the joining area, in a state of being in communication with the main inflation portion.