An airbag device for a front passenger's seat is made compact in a vehicle vertical direction. At an airbag device (10) for a front passenger's seat, due to an inflator (20) being fixed to a front side bottom wall portion (22F) that is formed at a vehicle front side and upper side of a rear side bottom wall portion (22E), the inflator (20) is offset toward the vehicle front side with respect to a central axis (L3) of a case (22). Further, the front side bottom wall portion (22F) is inclined with respect to the rear side bottom wall portion (22E) such that a central axis (L1) of the inflator (20) intersects a normal line (L2) of the rear side bottom wall portion (22E) further toward a vehicle upper side than airbag doors (12, 14), and due thereto, the inflator (20) is tilted. Accordingly, the airbag device (10) for a front passenger's seat can be made to be compact in a vehicle vertical direction as compared with a case in which, for example, the front side bottom wall portion (22F) is parallel to a direction in which the rear side bottom wall portion (22E) extends as shown by a two-dot chain line (L5).