An airbag for a front passenger's seat includes a left outer panel, right outer panel, left inner panel and right inner panel. The left and right outer panels are prepared as laterally symmetrical separate entities. Each of the left and right inner panels includes at the upper front end at full inflation a crossing edge that extends in a transverse direction on the airbag at full inflation. Each of the left and right outer panels includes at the upper periphery a central extended region that includes a joint edge for joint with the crossing edge of each of the inner panels. The central extended regions of the left and right outer panels are coupled together at their circumferences except the joint edges.