An embodiment air bag device includes a first chamber, wherein in a deployed state, the first chamber surrounds a first portion and a front portion of a seat with a frontal surrounding portion of the first chamber expanding toward a second portion, a second chamber, wherein in the deployed state, the second chamber surrounds the second portion and the front portion of the seat with a frontal surrounding portion of the second chamber expanding toward the first portion and overlaps with the first chamber behind the first chamber, and a first fastener provided at a portion where the first chamber and the second chamber overlap with each other and fastening the first chamber and the second chamber to each other.