A gas generator comprising: a housing having an elongated cylindrical shape and including a gas generating agent storing chamber storing a gas generating agent and a filter chamber storing a filter; a partition member inserted into said housing and partitioning a space within said housing into said gas generating agent storing chamber and said filter chamber in an axial direction; and an igniter installed in said housing and actuated to generate flame for burning said gas generating agent, said housing having a portion that defines said filter chamber and has a gas discharge opening for discharging gas, said filter being made of a member having a hollow portion extending in the axial direction of said housing and extending at least to an end face closer to said gas generating agent storing chamber, said partition member including an annular plate portion covering the end face of said filter closer to said gas generating agent storing chamber, a cylindrical portion continuously extending from an inner circumferential edge of said annular plate portion into said hollow portion of said filter and covering an inner circumferential surface of said filter closer to said end face, and a communication hole providing communication between said gas generating agent storing chamber and said filter chamber, wherein said communication hole provided in the partition member is only a single communication hole defined by an inner circumferential surface of said cylindrical portion and has an opening area changed in accordance with an increase in a distance from said annular plate portion characterized in that at least part of the annular plate portion of the partition member directly faces the gas generating agent storing chamber, without having another member interposed therebetween, in the axial direction of the housing such that the partition member defines an end of the gas generating agent storing chamber closer to the filter chamber. a housing having an elongated cylindrical shape and including a gas generating agent storing chamber storing a gas generating agent and a filter chamber storing a filter; a partition member inserted into said housing and partitioning a space within said housing into said gas generating agent storing chamber and said filter chamber in an axial direction; and an igniter installed in said housing and actuated to generate flame for burning said gas generating agent, said housing having a portion that defines said filter chamber and has a gas discharge opening for discharging gas, said filter being made of a member having a hollow portion extending in the axial direction of said housing and extending at least to an end face closer to said gas generating agent storing chamber, said partition member including an annular plate portion covering the end face of said filter closer to said gas generating agent storing chamber, a cylindrical portion continuously extending from an inner circumferential edge of said annular plate portion into said hollow portion of said filter and covering an inner circumferential surface of said filter closer to said end face, and a communication hole providing communication between said gas generating agent storing chamber and said filter chamber, wherein said communication hole provided in the partition member is only a single communication hole defined by an inner circumferential surface of said cylindrical portion and has an opening area changed in accordance with an increase in a distance from said annular plate portion characterized in that at least part of the annular plate portion of the partition member directly faces the gas generating agent storing chamber, without having another member interposed therebetween, in the axial direction of the housing such that the partition member defines an end of the gas generating agent storing chamber closer to the filter chamber.