To solve a problem of causing the weight increase of a gas generator since high generated pressure is demanded for a pressure vessel on the ignition side in the gas generator with pressure partition walls of two pressure vessels arranged opposedly to each other.

Both pressure partition walls 3, 5 of a first pressure vessel 11 charged with inflammable gas and a second pressure vessel 12 charged with gas are provided with fragile parts 30, 50 for accelerating cleavage, and it is so arranged that part of the pressure partition wall 3 of the first pressure vessel 11 after deforming both pressure partition walls 3, 5 into a cleaved state interferes with the fragile part 50 of the pressure partition wall 5 of the second pressure vessel 12. In a gas generator 1 of such constitution, the pressure partition wall 3 deformed into the cleaved state first is made to physically collide with the fragile part 50 of the other pressure partition wall 5. Two pressure partition walls can therefore be positively cleaved in a chain while reducing pressure generated by the first pressure vessel 11.