The present invention provides a gas generator, including: an ignition device being disposed at a first end of a cylindrical housing main body portion, a diffuser portion being provided with a gas discharge port and closing a second end thereof which is axially opposite to the first end; a combustion chamber being present in an internal space of the cylindrical housing main body portion; a cup-shaped member partitioning the combustion chamber and the diffuser portion, a bottom surface and an opening of the cup-shaped member being located on the side of the first end and the second end of the cylindrical housing main body portion respectively, and a first gap being formed between the circumferential wall of the cup-shaped member and an inner wall surface of the cylindrical housing main body portion; a cylindrical filter being disposed in a space surrounded by the cup-shaped member and the diffuser portion, and a first end surface of the cylindrical filter being abutted against the bottom surface of the cup-shaped member, and a second end surface thereof on the opposite side being abutted against a closed end surface of the diffuser portion; and the communication hole of the cup-shaped member being formed at a position closer to the first end surface of the cylindrical filter to face the first gap, and the gas discharge port of the diffuser portion being formed closer to the second end surface of the cylindrical filter.