A gas generator 88 (1A - 1T) comprising: a housing in a shape of a short cylinder, the housing being made up of a top plate part and a bottom plate part that close axial end parts, and a peripheral wall part provided with a gas discharge opening, and containing a combustion chamber accommodating a gas generating agent, an igniter attached to said bottom plate part, and containing an ignition part accommodating an ignition charge that ignites in actuation, a cup-shaped member in a shape of a substantial cylinder opening in an end part on a side of said bottom plate part, the cup-shaped member containing a transfer charge chamber accommodating a transfer charge, and disposed to project toward inside said combustion chamber in such a manner that said transfer charge chamber faces said ignition part, a cylindrical filter situated inside said housing, and disposed to surround said combustion chamber in a radial direction of said housing, and an annular first pad member situated between said bottom plate part and said gas generating agent in a part of said combustion chamber on the side of said bottom plate part, and applied to cover a boundary between said filter and said bottom plate part, said cup-shaped member having a top wall part and a lateral wall part defining said transfer charge chamber, said first pad member having an abutting part abutting with an inner circumferential face of an axial end part situated on the side of said bottom plate part of said filter, and a second extension part extending from said abutting part inwardly in the radial direction along the inner bottom face of said bottom plate part, characterized in that said cup-shaped member further comprises a first extension part extending outwardly in a radial direction along an inner bottom face of said bottom plate part from a part of said lateral wall part on a side of the open end, wherein by at least a part of said first extension part being arranged between said bottom plate part and said second extension part along an axial direction of said housing, movement of said cup-shaped member along the axial direction of said housing is restricted, and said cup-shaped member is assembled to said bottom plate part, and wherein between said second extension part and said bottom plate part of a part defining said combustion chamber, a space where said first extension part is not situated is formed. a housing in a shape of a short cylinder, the housing being made up of a top plate part and a bottom plate part that close axial end parts, and a peripheral wall part provided with a gas discharge opening, and containing a combustion chamber accommodating a gas generating agent, an igniter attached to said bottom plate part, and containing an ignition part accommodating an ignition charge that ignites in actuation, a cup-shaped member in a shape of a substantial cylinder opening in an end part on a side of said bottom plate part, the cup-shaped member containing a transfer charge chamber accommodating a transfer charge, and disposed to project toward inside said combustion chamber in such a manner that said transfer charge chamber faces said ignition part, a cylindrical filter situated inside said housing, and disposed to surround said combustion chamber in a radial direction of said housing, and an annular first pad member situated between said bottom plate part and said gas generating agent in a part of said combustion chamber on the side of said bottom plate part, and applied to cover a boundary between said filter and said bottom plate part, said cup-shaped member having a top wall part and a lateral wall part defining said transfer charge chamber, said first pad member having an abutting part abutting with an inner circumferential face of an axial end part situated on the side of said bottom plate part of said filter, and a second extension part extending from said abutting part inwardly in the radial direction along the inner bottom face of said bottom plate part, characterized in that said cup-shaped member further comprises a first extension part extending outwardly in a radial direction along an inner bottom face of said bottom plate part from a part of said lateral wall part on a side of the open end, wherein by at least a part of said first extension part being arranged between said bottom plate part and said second extension part along an axial direction of said housing, movement of said cup-shaped member along the axial direction of said housing is restricted, and said cup-shaped member is assembled to said bottom plate part, and wherein between said second extension part and said bottom plate part of a part defining said combustion chamber, a space where said first extension part is not situated is formed.