An air belt device (10) is provided with: a webbing (24) that fits onto a passenger (P) seated on a seat (12); and an air-bag (34) that is disposed along the webbing (24) and that expands and deploys when internally supplied with gas. The air-bag (34) is provided with a reference portion (S1) constituting a part of the air-bag (34) and formed by a folding of the air-bag (34). The air-bag (34) is also provided with a slack portion (S2) constituting another part of the air-bag (34) and formed by the folding of the air-bag (34), the slack portion extending along the reference portion (S1) and having a dimension in a width direction of the webbing (24) set greater than a dimension of the reference portion (S1) in the width direction of the webbing (24).
