A cylindrical housing is mounted with an igniter and a diffuser portion having a gas discharge port. A cylindrical member is disposed and fixed in the housing so as to form a cylindrical gap reaching the gas discharge port. An obstacle is provided in the gap, and a first gas passage hole, a second gas passage hole and a third gas passage hole are formed. A gas generating agent is ignited and burnt by activation of the igniter to generate gas, part of the generated gas flows into the gap through the first gas passage hole (f1), part of the gas that has passed therethrough flows again into the cylindrical member through the second gas passage hole (f2) to aid the combustion of the gas generating agent, and then the gas flows into the gap through the third gas passage hole (f3) and finally is discharged from the gas discharge port.