In the airbag device for front passenger's seat of the present invention, an airbag before being housed in the case and mounted on vehicle is folded up through a preparatory folding step and a regular folding step, the regular folding step further including a transverse folding step and a vertical folding step. In a preparatorily folding step, a portion near an upper edge of the passenger's side wall is located to confront the gas inlet port, and the passenger's side wall is flatly developed so as to overlap with a lower part of the circumferential wall. In the transverse folding step, a portion of the preparatorily folded airbag located rearward of the gas inlet port is folded back in two toward a lower side of a lower part of the circumferential wall to bring a rear end thereof closer to the gas inlet port. The airbag device of the present invention shifts an entire rear face of the deploying airbag rearward in a generally vertical condition.