An airbag system for a front passenger's seat of the top-mount type according to the invention is arranged in an instrument panel below a windshield. The front passenger's seat airbag system is constructed to comprise a door, a case and an airbag. The door is arranged on the upper face of the instrument panel. The case houses and holds the airbag in a folded state. The airbag, as housed in the case, is expanded by injecting an inflating gas into a gas inlet port so that the airbag protrudes to open the door and towards the rear side of a vehicle along the windshield. The airbag is equipped with a commutator cloth arranged to cover the gas inlet port. This commutator cloth is arranged to close the two sides in the transverse direction of the vehicle and to open the two sides in the longitudinal direction of the vehicle. This front passenger's seat airbag system reduces the expansion rate of the airbag towards the rear of the vehicle.