The present invention provides a gas generator including a cylindrical housing in which an ignition device is attached at a first end portion thereof and a diffuser portion provided with a gas discharge port is attached at a second end portion thereof, an annular end surface being formed on the side of the second end portion, a cylindrical flow path forming member being disposed in the cylindrical housing, an inner space of the cylindrical flow path forming member being partitioned by a partition wall into a first chamber on the side of the first end portion and a second chamber on the side of the second end portion, a space, which includes the first chamber, being a combustion chamber filled with a gas generating agent, and the second chamber forming part of a gas discharge path, in the cylindrical flow path forming member, a first cylindrical space, which is formed between a first wall surface of the cylindrical flow path forming member on the side of the first chamber and the inner wall surface of the cylindrical housing, and a second cylindrical space, which is formed between a second wall surface of the cylindrical flow path forming member on the side of the second chamber and the inner wall surface of the cylindrical housing, being continuous in an axial direction, and the second cylindrical space being a cylindrical space with a length L from a boundary with the first cylindrical space to the annular end surface, and having an annular pocket portion with a length L1 from the annular end surface to the second communication holes, and at least part of a surface of the annular pocket portion being provided with a concave-convex portion.