Gas generator for air bag

The present invention provides a gas generator (100) for an air bag that can be reduced in size and thickness. A gas outlet (21) is formed in the axial direction of the gas generator (100) in a diffuser (20) which is attached to an open end portion of a bottle portion (10) storing a pressurized medium (11). The gas outlet (21) is closed by a rupturable plate (22), and an igniter collar (27) attached with an igniter (26) for rupturing the rupturable plate (22) is attached to the diffuser (20). A first gas discharge hole (32) that opens in the axial direction of a housing is also formed in the igniter collar (27), and hence no member protrudes in the width direction of the gas generator, enabling the entire gas generator to be reduced in size and thickness.