An air bag system having an air bag module 20 including an inflator 22 and an air bag 25 is installed in a seat back 12. When a side collision occurs against a vehicle, the air bag 25 is inflated between a passenger and an inner wall of a vehicle compartment. The air bag 25 has a separator 27 for vertically separating the inside of the air bag 25 into first and second chambers 28 and 29. The separator 27 is extended along a vehicle front-to-rear direction, and defines a communication portion 27a at a vehicle rear side in the air bag 25 for interconnecting the two chambers 28 and 29 and guiding gas blown by the inflator 22 from the first chamber 28 to the second chamber 29.
