A gas generator, comprising: a tubular housing having both ends (12a, 12b) closed and a gas discharge port (116a, 116b, 116c) on a circumferential surface, and forming an outer shell of the gas generator (10; 100), wherein a ratio (L/D) of an outer diameter and a length of the tubular housing exceeds 1; a first closing member having an ignition device fixed thereto and closing one end of the tubular housing; a second closing member closing another end of the tubular housing; a combustion chamber defined inside the tubular housing and charged with a gas generating agent, the gas discharge port (116a, 116b, 116c) including at least two gas discharge port groups (116a, 116b, 116c), the at least two gas discharge port groups (116a, 116b, 116c) including a first gas discharge port group formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, and a second gas discharge port group formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, pressure (opening pressure P 1), that is necessary for opening the first gas discharge port group, and pressure (opening pressure P 2), that is necessary for opening the second gas discharge port group, satisfying a relationship of P 1 < P 2, the second gas discharge port group being separated axially from the first gas discharge port group, the first gas discharge port group being positioned at a center portion in the axial direction between the first closing member and the second closing member, or in the vicinity thereof, and the second gas discharge port group being positioned at a center portion in the axial direction between the first gas discharge port group and the second closing member, or in the vicinity thereof, wherein a tubular filter is arranged inside the tubular housing so as to obtain a gap between the tubular filter and an inner peripheral wall of the tubular housing; and the interior of the tubular housing corresponds to the combustion chamber characterized in that the gas generating agent is charged inside the tubular filter, and in that in the tubular filter, an end surface abuts against the second closing member and an end surface is supported by a supporting member that is a combination of a retainer and a spacer, wherein the retainer includes an annular plane surface, an annular inner circumferential surface extending in the axial direction of the tubular housing from an inner circumferential edge of the annular plane surface, and an annular outer circumferential surface extending in the direction opposite to the annular inner circumferential surface from an outer circumferential edge of the annular plane surface. a tubular housing having both ends (12a, 12b) closed and a gas discharge port (116a, 116b, 116c) on a circumferential surface, and forming an outer shell of the gas generator (10; 100), wherein a ratio (L/D) of an outer diameter and a length of the tubular housing exceeds 1; a first closing member having an ignition device fixed thereto and closing one end of the tubular housing; a second closing member closing another end of the tubular housing; a combustion chamber defined inside the tubular housing and charged with a gas generating agent, the gas discharge port (116a, 116b, 116c) including at least two gas discharge port groups (116a, 116b, 116c), the at least two gas discharge port groups (116a, 116b, 116c) including a first gas discharge port group formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, and a second gas discharge port group formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, pressure (opening pressure P 1), that is necessary for opening the first gas discharge port group, and pressure (opening pressure P 2), that is necessary for opening the second gas discharge port group, satisfying a relationship of P 1 < P 2, the second gas discharge port group being separated axially from the first gas discharge port group, the first gas discharge port group being positioned at a center portion in the axial direction between the first closing member and the second closing member, or in the vicinity thereof, and the second gas discharge port group being positioned at a center portion in the axial direction between the first gas discharge port group and the second closing member, or in the vicinity thereof,