A gas generator for a restraining device includes, a housing having an ignition device and a diffuser portion, and a gas generating agent charged in an inner space of the housing, a partition member, having a communication hole, and inclining with respect to a longitudinal axis (X) of the housing, thereby dividing the inner space into a gas generating agent charging space and a gas flow channel space, the gas flow channel space communicating with the diffuser portion, both of the edges of the partition member abutting against an inner circumferential surface of the housing, a cross-sectional area of the gas generating agent charging space decreasing from the ignition device side at one end of the housing to the diffuser portion side at the other end of the housing, and a cross-sectional area of the gas flow channel space increasing from the ignition device side to the diffuser portion side.