To generate predetermined tension in an air belt even if an expandable part is expanded in a narrow space.

The air belt 30 has a first normal belt 31, a second normal belt 32, and the expandable part 33 between them. To regulate a thickness at the expanding of the expandable part 33, an opposed part of the expandable part 33 is connected with each other by a connecting part 34 composed of suture thread. The expandable part 33 has a baglike belt 33a and a knit cover 33b not elongating in the longitudinal direction. When an inflator is operated, the expandable part 33 is expanded, and an air belt 30 is fitted to an occupant. In this case, since the knit cover of the expandable part 33 is not elongated in the longitudinal direction of the air belt 30, the length of the expandable part 33 is shortened accompanied by the expansion of the expandable part 33, and the tension is generated in the air belt 30.

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