There are provided an airbag folding device, and an airbag folding method, for mechanically folding an airbag to form a doughnut-shaped airbag. The airbag folding device is characterized in comprising a support member (62) columnar in shape, rising while supporting a tubular tether belt (11a), an outer cylinder (63) coaxial with the support member (62), for containing the airbag body (10a) on the outside of the support member (62), by rising, a work table (61) having the support member (62), and the outer cylinder (63), and a clamping member (64) positioned straight above the support member (62), for clamping the tubular tether belt (11a) between the clamping member (64) and the support member (62), wherein a pair of plates (65) in such a state as to cover an upper end of the outer cylinder (63), and to be fitted to a lower end of the clamping member (64) cause the support member (62), the outer cylinder (63) and the clamping member (64) to concurrently descend.