A gas generator (10; 100; 200) comprising: a cylindrical housing having both ends (12a, 12b) closed, a gas discharge port in a peripheral surface thereof and a combustion chamber charged with a gas generating agent therein, one end of the tubular housing being closed by a closing member having an igniter fixed thereto, the closing member comprising a disk-shaped main body portion, an annular projection portion formed in a peripheral surface at one side of the main body portion, and the peripheral surface of the annular projection portion abutting against the inner peripheral surface of the tubular housing, a transfer charge chamber housing defining a transfer charge chamber charged with a transfer charge, the transfer charge chamber housing attached to the closing member so as to surround the igniter, the transfer charge chamber housing having a cup-like shape including a peripheral surface and a bottom surface formed with a flame-transferring hole, and including a flange portion formed at an opening portion thereof, characterized by the transfer charge chamber housing further including an annular wall surface extending in the axial direction of the housing from the flange portion, the transfer charge chamber housing being mounted such that the annular wall surface thereof is fitted into a part of an annular groove defined by the tubular housing, the main body and the annular projection portion of the closing member, and the flange portion abuts against the surface of the main body, the transfer charge chamber housing being fixed by the flange portion pressed in the axial direction by a supporting member (20; 20, 80, 90) inside the tubular housing, and a sealing member being disposed in a remaining space of the annular groove and sealing between the tubular housing and the closing member. a cylindrical housing having both ends (12a, 12b) closed, a gas discharge port in a peripheral surface thereof and a combustion chamber charged with a gas generating agent therein, one end of the tubular housing being closed by a closing member having an igniter fixed thereto, the closing member comprising a disk-shaped main body portion, an annular projection portion formed in a peripheral surface at one side of the main body portion, and the peripheral surface of the annular projection portion abutting against the inner peripheral surface of the tubular housing, a transfer charge chamber housing defining a transfer charge chamber charged with a transfer charge, the transfer charge chamber housing attached to the closing member so as to surround the igniter, the transfer charge chamber housing having a cup-like shape including a peripheral surface and a bottom surface formed with a flame-transferring hole, and including a flange portion formed at an opening portion thereof, characterized by the transfer charge chamber housing further including an annular wall surface extending in the axial direction of the housing from the flange portion, the transfer charge chamber housing being mounted such that the annular wall surface thereof is fitted into a part of an annular groove defined by the tubular housing, the main body and the annular projection portion of the closing member, and the flange portion abuts against the surface of the main body, the transfer charge chamber housing being fixed by the flange portion pressed in the axial direction by a supporting member (20; 20, 80, 90) inside the tubular housing, and a sealing member being disposed in a remaining space of the annular groove and sealing between the tubular housing and the closing member.