A gas generator comprising: a tubular housing having both ends (212a, 212b) closed and a gas discharge port (216a, 216b, 216c, 218) on a circumferential surface, and forming an outer shell of the gas generator, wherein a ratio (L/D) of a diameter and a length of the tubular housing of a portion that forms a first combustion chamber 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, and a partition member dividing a space between the first closing member and the second closing member, among spaces (270a, 270b) divided by the partition member, a space that is closer to the first closing member and the ignition device, serving as the first combustion chamber charged with a first gas generating agent, a space that is closer to the second closing member, serving as a second combustion chamber charged with a second gas generating agent, the spaces (270a, 270b) satisfying a relationship where capacity of the first combustion chamber > capacity of the second combustion chamber, the gas discharge port (216a, 216b, 216c), that is formed in a side of the first combustion chamber, including at least two gas discharge port groups (216a, 216b, 216c), the at least two gas discharge port groups (216a, 216b, 216c) 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 separated axially from the first gas discharge port group, formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, characterized in that pressure, that is necessary for opening the first gas discharge port group, and pressure, that is necessary for opening the second gas discharge port group, satisfying a relationship of P1 < P2, the first gas discharge port group being positioned at a center portion in the axial direction or in the vicinity thereof between the first closing member and the partition member, and the second gas discharge port group being positioned at a center portion in the axial direction or in the vicinity thereof between the first gas discharge port group and the partition member. a tubular housing having both ends (212a, 212b) closed and a gas discharge port (216a, 216b, 216c, 218) on a circumferential surface, and forming an outer shell of the gas generator, wherein a ratio (L/D) of a diameter and a length of the tubular housing of a portion that forms a first combustion chamber 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, and a partition member dividing a space between the first closing member and the second closing member, among spaces (270a, 270b) divided by the partition member, a space that is closer to the first closing member and the ignition device, serving as the first combustion chamber charged with a first gas generating agent, a space that is closer to the second closing member, serving as a second combustion chamber charged with a second gas generating agent, the spaces (270a, 270b) satisfying a relationship where capacity of the first combustion chamber > capacity of the second combustion chamber, the gas discharge port (216a, 216b, 216c), that is formed in a side of the first combustion chamber, including at least two gas discharge port groups (216a, 216b, 216c), the at least two gas discharge port groups (216a, 216b, 216c) 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 separated axially from the first gas discharge port group, formed in the circumferential direction of the tubular housing and closed, from the inside, by a seal member, characterized in that pressure, that is necessary for opening the first gas discharge port group, and pressure, that is necessary for opening the second gas discharge port group, satisfying a relationship of P1 < P2, the first gas discharge port group being positioned at a center portion in the axial direction or in the vicinity thereof between the first closing member and the partition member, and the second gas discharge port group being positioned at a center portion in the axial direction or in the vicinity thereof between the first gas discharge port group and the partition member.