The present invention provides a gas generator, including: a housing provided with a gas discharge port; an igniter assembly and a gas generating agent accommodated in the housing; a metallic cup member having a constant inner diameter and covering the igniter assembly to define an agent-charging chamber; in the igniter assembly, an igniter main body, which includes an ignition portion and an electro-conductive pin, being integrated with a metallic collar by a resin; the metallic collar having, a fixing portion including a portion inserted into a hole in a bottom surface of the housing and a remaining portion projecting to an inner side of the bottom surface of the housing, and a guide portion formed axially in connection with the fixing portion, the guide portion having an inclined surface in which an outer diameter (D3) of a portion in contact with the fixing portion is a maximum diameter and an outer diameter (D4) of a portion on the axially opposite side is a minimum diameter; the metallic cup member and the metallic collar satisfying D1 = D3, where (D1) is an outer diameter of the fixing portion and (D3) is a maximum outer diameter of the guide portion, D �¢ 1 = D �¢ 3 ‰¥ R �¢ 1 , where (R1) is an inner diameter of the metallic cup member, (D1) is an outer diameter of the fixing portion and (D3) is a maximum outer diameter of the guide portion and R1 > D4 where (D4) is a minimum outer diameter of the guide portion and (R1) is an inner diameter of the metallic cup member; and the metallic cup member being fixed in a state in which an inner circumferential wall surface of the metallic cup member and an outer circumferential wall surface of the fixing portion are press-fitted, and an annular gap is formed between the inner circumferential wall surface of the metallic cup member and an outer circumferential wall surface of the guide portion.