Abstract: In an air bag device 1 against a side collision which can be expanded and spread out in a gap between an outside portion of a seat of a vehicle and a wall opposite thereto owing to gas generated by an inflator 20 in emergency situations so as to obtain the air bag device at a low cost which system is provided with plural air chambers having difference in expansion timing and pressure depending on portions of a body of a crew, an air bag 10 is structured by a lower air chamber 12 provided with the inflator and an upper air chamber 14, a partition unit 40 for partitioning the lower air chamber 12 and the upper air chamber 14 is structured by a cloth sewn to the air bag, and a communication hole 40b which introduces gas from the inflator 20 from the lower air chamber 14 into the upper air chamber and is closed when subjected to pressure of the upper chamber as the pressure of the lower air chamber is reduced is formed in the cloth.