At an airbag device for a front passenger's seat, due to an inflator being fixed to a front side bottom wall portion that is formed at a vehicle front side and upper side of a rear side bottom wall portion, the inflator is offset toward the vehicle front side with respect to a central axis of a case. Further, the front side bottom wall portion is inclined with respect to the rear side bottom wall portion such that a central axis of the inflator intersects a normal line of the rear side bottom wall portion further toward a vehicle upper side than airbag doors, and due thereto, the inflator is tilted.