Abstract: The present invention relates to a gas generator for an air bag apparatus mounted in a vehicle or the like for protecting an occupant in the vehicle by charging gas into the air bag interposed between the occupant and a vehicle member at the time of collision of the vehicle. The gas generator for the air bag of the present invention comprises a first chamber (9) accommodating a flammable fluid (7) and having a first vent opening (8) which is in communication with the air bag (3), a second chamber (5) accommodating the flammable fluid (7) and having a volume greater than that of the first chamber (9), a first occluding member (31) for occluding the first vent opening (8), and ignition means (14) for igniting the flammable fluid (7) in the first chamber (9). By burning at least a portion of the flammable fluid (7) in the first chamber (9), the fluid in the first chamber (9) and the second chamber (5) is discharged out through the first vent opening (8), thereby inflating the air bag (3).