A predetermined tension force is generated to an air belt even when an expandable portion is expanded in a narrow space. An air belt 30 includes a first normal belt 31, a second normal belt 32, and an expandable portion 33 therebetween. In order to regulate a thickness of the expandable portion 33 in an expanding operation, portions of the expandable portion 33 facing each other are coupled by means of a coupling portion 34 formed of a stitching thread. The expandable portion 33 includes a bag-shaped belt 33a and a knit cover 33b that is not extended in a longitudinal direction. When an inflator is operated, the expandable portion 33 is expanded and the air belt 30 fits an occupant. At this moment, since the knit cover of the expandable portion 33 is not extended in the longitudinal direction of the air belt 30, a length of the expandable portion 33 is reduced along with an expansion of the expandable portion 33 and the tension force is generated to the air belt 30.