A gas generator (1A; 1B; 1C; 1D; 1G; 1H; 11; 1J; 1K) comprising: an elongated cylindrical housing containing a combustion chamber accommodating a gas generating agent and a filter chamber accommodating a filter and having opposing ends closed; a partition member (41; 50) inserted in the housing and axially dividing a space in the housing into the combustion chamber and the filter chamber; and an igniter assembled to the housing, for burning the gas generating agent, the housing having in a portion defining the filter chamber, a gas discharge opening for discharging a gas to outside, the filter being formed from a member having a hollow portion extending along an axial direction of the housing and reaching an axial end surface located on a side of the combustion chamber, the partition member (41; 50) having a partition portion (42; 51) opposed to the axial end surface of the filter, the partition portion (42; 51) including a first region (42a; 51a) which opens as a result of burning of the gas generating agent as being located to be opposed to the hollow portion and a second region (42b; 51b) which does not open in spite of burning of the gas generating agent as being annularly located to surround the first region (42a; 51a) so as to be opposed to a portion of the axial end surface of the filter except for the hollow portion, and the second region (42b; 51b) being provided with a large-thickness portion constructed to be greater in thickness than the first region (42a; 51a) and at least opposed to an outer edge portion of the axial end surface of the filter, characterized in that the large-thickness portion is constructed to gradually be greater in thickness from an inner circumferential end portion toward an outer circumferential end portion. an elongated cylindrical housing containing a combustion chamber accommodating a gas generating agent and a filter chamber accommodating a filter and having opposing ends closed; a partition member (41; 50) inserted in the housing and axially dividing a space in the housing into the combustion chamber and the filter chamber; and an igniter assembled to the housing, for burning the gas generating agent, the housing having in a portion defining the filter chamber, a gas discharge opening for discharging a gas to outside, the filter being formed from a member having a hollow portion extending along an axial direction of the housing and reaching an axial end surface located on a side of the combustion chamber, the partition member (41; 50) having a partition portion (42; 51) opposed to the axial end surface of the filter, the partition portion (42; 51) including a first region (42a; 51a) which opens as a result of burning of the gas generating agent as being located to be opposed to the hollow portion and a second region (42b; 51b) which does not open in spite of burning of the gas generating agent as being annularly located to surround the first region (42a; 51a) so as to be opposed to a portion of the axial end surface of the filter except for the hollow portion, and the second region (42b; 51b) being provided with a large-thickness portion constructed to be greater in thickness than the first region (42a; 51a) and at least opposed to an outer edge portion of the axial end surface of the filter, characterized in that the large-thickness portion is constructed to gradually be greater in thickness from an inner circumferential end portion toward an outer circumferential end portion.