An airbag is provided which is folded with a duct part and an expansion part, the duct part having an inlet for allowing fluid communication throughout the duct part, the expansion part communicating with the duct part, wherein the airbag comprises an aggregative part obtained by aggregating a downward portion of the airbag, and a plurality of arms obtained by folding an upward portion of the airbag and being in fluid communication along the length direction of the airbag. The arms cover the aggregative part on the back thereof with respect to the expansion direction of the airbag. Thus, even having greater length from one end to another end, the airbag can deploy smoothly with less time difference between the front and the back in the length direction.