PURPOSE: To achieve uniform pressure pass in a gas generator in which a discharge opening for generated gas is gas-tightly closed with a cover that can be broken by setting the total cross sectional surface of the discharge opening to automatically increase with relation to increasing combustion chamber pressure.

CONSTITUTION: A gas generator has a combustion chamber 2 to contain fuel 1, this combustion chamber 2 is connected through an opening 3 radially outward to a diffuser chamber 4 continuous with it, and this diffuser chamber 4 is further connected through an opening 5 to the inside of an air bag. The opening 3 between the combustion chamber 2 and the diffuser chamber 4 is gas-tightly closed with a cover 7 applied to the inside of a wall 6 in the form of metal foil, for example, that can be broken when proper pressure is generated in a manufactured condition. In this case, the opening 3 of the same size is provided in the wall 6, and a specified breaking line 9 starting from a breakage range on the back to be extended radially is set to be broken only by higher pressure.