Method of assembling airbag device, and airbag device

Efficiency of assembling an airbag device to an airbag cover (16) is improved. The air bag device is assembled in the following steps: an airbag (10) integrally connected to the forward end and rear end of a tubular tether belt (11a) is stretched to the position of its full length with the peripheral edge of an inflator installation opening of the airbag (10) fixed to a cushion ring (22), the tubular tether belt (11a) is pulled back while being stretched and the airbag (10) is folded back around the periphery of the tubular tether belt (11a), the tubular tether belt (11a) and the airbag (10) are folded while being pulled back, and the forward end of the folded tether belt (11a) is fixed to the center of the back face of the cover (16) for receiving the airbag (10).