Abstract: An air bag device is structured in such a manner that an air bag can expand entirely and uniformly in a three-dimensional manner from the center thereof in the radial directions and in the forward direction of the air bag device all through the process in which the air bag inflates to unfold. The flat, unfolded air bag is folded from both ends thereof to form an elongated folding body, and the elongated folding body is folded from both longitudinal-directional ends thereof toward the side opposite to that where gas ejecting holes of the inflator are formed in such a manner as to wrap these ends inside, and a pair of roll-shaped folding bodies are thereby formed. Further, a preceding expanding and first folding portion provided to expand first is formed in a space between the pair of roll-shaped folding bodies.