An airbag with an inflation gas diffuser comprising an inflator side cloth and an occupant side cloth both of a substantially equal size and a generally circular shape seamed together at outer peripheral portions thereof to form an outer configuration of the airbag. The inflator side cloth has a central opening through which to communicate with an inflator, and an internal cloth of a smaller size enclosed within the airbag to cover the opening. The internal cloth is partially seamed at its outer perimetric portion, to the inflator side cloth so as to form circumferentially alternately seamed portions and non-seamed open portions acting as a control passage for the inflation gas. The internal cloth may be circular, semicircular, dumbbell, polygonal, etc. The seamed portions each may consist of a main straight seam and two curved end seams.