Provided is a gas generator which is reduced in weight and size while maintaining a sufficient strength as a pressure vessel. A gas generator (1) includes a housing (4) in a tubular shape having an opening (10) on at least one end thereof and a plug (12) inserted in the opening (10), so that an end portion (11) on the opening side is reduced in diameter and caulked in a caulking groove (13) of the plug (12), to thereby seal the opening (10), in which: the caulking groove (13) is a streak groove that extends along the periphery of the plug (12) and is rectangular in section; and the caulking groove (13) has at least one of side walls (14) forming the calking groove (13), the one side wall (14a) being closer to the center of the housing, which is orthogonal or sloped to the opening side, with respect to the axis (X) of the housing (4).