The air bag gas generator comprises a housing having a gas discharge port, an ignition unit chamber in which ignition units are accommodated, and a combustion chamber in which a gas generating agent is accommodated, a cylindrical coolant/filter disposed between the combustion chamber and the gas discharge port, so that combustion gas passes through the coolant/filter prior to discharge through the gas discharge port, and the air bag gas generator meeting the following requirements (a), (b), and (c):(a) that the gas generating agent has an amount of heat generation of 10 to 30 kcal/mol;(b) that the mass (g) of the coolant/filter in relation to 1 mol of gas generated during combustion of the gas generating agent be 20 to 65 g/mol; and(c) that the coolant/filter has a thickness of not less than 4.0 mm.