A gas generator includes a substantially cylindrical cup with bottom where a gas generating agent is accommodated, the cup including one axial end as an opening end, and a substantially columnar holder to which the cup is coaxially assembled to close the opening end, the holder holding an igniter such that an ignition portion faces the gas generating agent. An annular step portion is provided at a periphery of an axial end of the holder on a side facing the cup. The cup is fixed to the holder by fitting the opening end to the annular step portion and providing an annular welded portion at the opening end, the annular welded portion being joined to the annular step portion so as to extend along a circumferential direction of the cup.