There is provided an airbag folding method for folding an airbag. The method includes in the following order: (a) folding the inner panel and the outer panel into a symmetrical substantially rectangular shape such that a groove of the concave region is located along a center line, wherein the center line passes through a center of the introduction port and symmetrically divides the inner and outer panels; (b) folding the inner and outer panels along fold lines from right and left edges thereof toward the center line, wherein the fold lines are substantially parallel to the center line; and (c) folding and rolling up the inner and outer panels from both ends of the center line toward the introduction port.