A gas generator, comprising: a housing provided with a gas discharge port; an igniter assembly (21, 121) and a gas generating agent accommodated in the housing; a metallic cup member having a constant inner diameter and covering the igniter assembly (21, 121) to define an agent-charging chamber; in the igniter assembly (21, 121), 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 (50, 150) 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 (54, 154) formed axially in connection with the fixing portion (50, 150) between the fixing portion (50, 150) and the resin, the guide portion (54, 154) having an inclined surface in which the outer diameter of a portion in contact with the fixing portion (50, 150) is the maximum diameter and the outer diameter of a portion on the axially opposite side is the minimum diameter; 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 (50, 150) are press-fitted, and an annular gap (52, 152) is formed between the inner circumferential wall surface of the metallic cup member and an outer circumferential wall surface of the guide portion (54, 154), the metallic cup member and the metallic collar being made from metals that are different in hardness, the metallic cup member and the metallic collar satisfying before being press-fitted together, D1 = D3, where is the outer diameter of the fixing portion (50, 150) and is the maximum outer diameter of the guide portion, D1 = D3 > R1, where is the inner diameter of the metallic cup member, R1 > D4 where is the minimum outer diameter of the guide portion; D4/D3 = 0.96 to 0.99. a housing provided with a gas discharge port; an igniter assembly (21, 121) and a gas generating agent accommodated in the housing; a metallic cup member having a constant inner diameter and covering the igniter assembly (21, 121) to define an agent-charging chamber; in the igniter assembly (21, 121), 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 (50, 150) 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 (54, 154) formed axially in connection with the fixing portion (50, 150) between the fixing portion (50, 150) and the resin, the guide portion (54, 154) having an inclined surface in which the outer diameter of a portion in contact with the fixing portion (50, 150) is the maximum diameter and the outer diameter of a portion on the axially opposite side is the minimum diameter; 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 (50, 150) are press-fitted, and an annular gap (52, 152) is formed between the inner circumferential wall surface of the metallic cup member and an outer circumferential wall surface of the guide portion (54, 154), the metallic cup member and the metallic collar being made from metals that are different in hardness, the metallic cup member and the metallic collar satisfying before being press-fitted together, D1 = D3, where is the outer diameter of the fixing portion (50, 150) and is the maximum outer diameter of the guide portion, D1 = D3 > R1, where is the inner diameter of the metallic cup member, R1 > D4 where is the minimum outer diameter of the guide portion; D4/D3 = 0.96 to 0.99.