An airbag device for a front passenger's seat, comprising: an airbag door (12, 14) provided at a vertical direction intermediate portion that is formed curvingly so as to be convex toward a vehicle upper side and rear side between an upper portion and a lower portion of an instrument panel; and an airbag that is provided in a folded-up state at a reverse side of the airbag door (12, 14), and that, when the airbag receives a supply of gas, opens the airbag door (12, 14) and is inflated and expanded at an obverse side of the instrument panel; characterized by an inflator that is disc-shaped and is provided at the reverse side of the airbag door (12, 14) and supplies gas to the airbag; and by a front side accommodating portion together with a rear side accommodating portion that structure a box-shaped case that is open at the airbag door side, wherein the front side accommodating portion accommodates a general portion of the airbag and the inflator, in that the rear side accommodating portion has a rear side bottom wall portion that is provided at the reverse side of the airbag door (12, 14) and faces the airbag door side, and that accommodates a lower inflating portion of the airbag, that is positioned at an obverse side of a lower portion of the instrument panel, at the airbag when inflation is completed; and in that the front side accommodating portion has a front side bottom wall portion that is formed at a vehicle front side and upper side of the rear side bottom wall portion, and to which are fixed the inflator and a base portion (18B1) of the general portion of the airbag that is other than the lower inflating portion of the airbag, and which 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 the airbag door (12, 14). an airbag door (12, 14) provided at a vertical direction intermediate portion that is formed curvingly so as to be convex toward a vehicle upper side and rear side between an upper portion and a lower portion of an instrument panel; and an airbag that is provided in a folded-up state at a reverse side of the airbag door (12, 14), and that, when the airbag receives a supply of gas, opens the airbag door (12, 14) and is inflated and expanded at an obverse side of the instrument panel; characterized by an inflator that is disc-shaped and is provided at the reverse side of the airbag door (12, 14) and supplies gas to the airbag; and by a front side accommodating portion together with a rear side accommodating portion that structure a box-shaped case that is open at the airbag door side, wherein the front side accommodating portion accommodates a general portion of the airbag and the inflator, in that the rear side accommodating portion has a rear side bottom wall portion that is provided at the reverse side of the airbag door (12, 14) and faces the airbag door side, and that accommodates a lower inflating portion of the airbag, that is positioned at an obverse side of a lower portion of the instrument panel, at the airbag when inflation is completed; and in that the front side accommodating portion has a front side bottom wall portion that is formed at a vehicle front side and upper side of the rear side bottom wall portion, and to which are fixed the inflator and a base portion (18B1) of the general portion of the airbag that is other than the lower inflating portion of the airbag, and which 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 the airbag door (12, 14).