A gas generator for an air bag, which can ensure a reliable activation and an operation performance as designed without adding further members. The gas generator includes a moving body for rupturing a first rupturable plate that closes a first opening provided between a first gas generating chamber and a pressurized medium accommodating chamber, and a second opening and a second rupturable plate for closing the second opening provided at a position deviated from a hitting portion of the moving body rupturing the first rupturable plate.