PROBLEM TO BE SOLVED: To provide an airbag device capable of suitably adjusting the length of a supporting belt at the time of expansion/deployment of an airbag. SOLUTION: At the time of the expansion/deployment of the airbag 40, the supporting belt 10 for supporting the airbag 40 from a rear side and a lower side of a vehicle body of a scooter type motorcycle 1 is made to be shorter than that before the expansion/deployment by actuating a wind-up unit 30 as a belt length adjusting means. Although the supporting belt 10 becomes greatly longer than suitable length for supporting the airbag 40 when stored in a storage groove formed along a foot placing portion 12 of an occupant, the supporting belt 10 is wound up by the wind-up unit 30 to be suitable length, thereby showing a function holding the airbag 40 at a predetermined position. To the wind-up unit 30, a mechanical type rotating a retractor by gas generated from an inflator or an electric type rotating the retractor by an actuator driven by electric power is applied. COPYRIGHT: (C)2008,JPO&INPIT