PROBLEM TO BE SOLVED: To provide a bag within a bag type air bag which can be effectively and economically manufactured by a mass production technology and to provide the air bag capable of controlling a gas current between an inner air bag and an outer air bag of the air bag. SOLUTION: An air bag 40 includes an outer air bag 40A having an inside cavity and an inlet. An inner air bag having an inlet is arranged in the inside cavity of the outer air bag 40A, and the inlet of the outer air bag 40A and the inlet of the inner air bag 40B are fixed on a car body at a common point. The inner air bag 40B is an integrally structured member having at least one gas flow control device (for example, a ventilating hole, a low strength part to be broken at the time of development).