An airbag according to the present invention has: an airbag main body that is provided with a gas introduction opening; and an inner cloth portion that is arranged to cover the gas introduction opening in the airbag main body. The inner cloth portion has a first and a second opening end parts, and is fixed to a peripheral edge part of the gas introduction opening of the airbag main body while a position of the inner cloth portion is superimposed with the gas introduction opening of the airbag main body. In this manner, an airbag is caused to inflate and expand more speedily and widely in a direction which is orthogonal to an occupant's side direction, and an occupant can be buffered and supported in a stable manner so as to have a wide area from a front side irrespective of whatsoever posture seated by the occupant may be.