The airbag device for front passenger's seat is mounted on the dashboard in front of the front passenger's seat. The airbag device for front passenger's seat includes an airbag to be inflated with the inflation gas, and an inflator for feeding the airbag with the inflation gas of pyro- and long mass flow type. The gas permeability rate of the airbag is 1.0 l/cm.sup.2/min or under at 20 kPa air pressure, and its weight is 230 g/m.sup.2 or under. This airbag device for front passenger's seat prevents the gas leakage, expands and inflates the airbag properly, and constitutes to the weight reduction.