An airbag device, provided below a pad of a vehicle seat cushion, comprises an airbag which deploys at a time of a vehicle collision, and an enveloping member which envelops the airbag in a tube shape; the airbag device is characterized in being provided at a position substantially in a center of the seat cushion in the back-and-forth direction of the vehicle, and in that the enveloping member has different peripheral lengths in an intermediate portion thereof and in end portions thereof.