The inflation and deployment speed of an airbag of a knee-protecting airbag device is increased to quickly deploy the airbag to a position where it can protect the knees of the occupant, thereby improving the occupant protection function. An airbag (10), from a state in which it is arranged in a flat form, is folded sequentially through a first step in which a leading end (10S) is tucked and folded into the airbag (10), a second step in which left and right ends (10M) of the airbag (10) are roll-folded toward each other toward a vehicle-body-side base fabric sheet (12) on the vehicle-body side, and a third step in which the leading end side of the airbag (10) in the deploying direction (F) is roll-folded toward the base end (10T) toward the vehicle-body-side base fabric sheet (12). The folded airbag (10) is accommodated in a case together with an inflator (2), and a base end (10T) thereof is fixed to the case. Thus, an airbag device (1) is produced and is attached to a vehicle, at a position in front of the knees of the occupant.