An air belt device is provided with a webbing that is fitted around a vehicle occupant who is sitting in a vehicle seat, and an airbag portion that is provided in parallel with the webbing and is inflated and deployed when gas is supplied to an interior thereof. The airbag is provided with a reference portion that forms a portion of the airbag and is formed as a result of the airbag being folded. The airbag is also provided with a slack portion that forms another portion of the airbag and is formed as a result of the airbag being folded, and that extends in parallel with the reference portion and whose dimension in the transverse direction of the webbing is set longer than a dimension in the transverse direction of the webbing of the reference portion.