To provide a side air bag device allowing an air bag to be easily folded and to be easily and swiftly arranged at the side of an occupant from the beginning of expansion.

In this side air bag device, the folded air bag 17 is expandably stored in the vehicle outer-side side face of a seat back. The air bag 17 is provided with a base part 25 positioned on the upper reaches of expanding gas when expanded, and a body part 26 arranged in front of the base part 25 when expanded. The body part 26 is provided with a lower expansion part 28 arranged in front of the base part 25 when expanded, and an upper expansion part 27 communicating with the upper part of the lower expansion part 28. In the flat expanded state of the air bag 17, the upper expansion part 27 is folded into the lower expansion part 28. With a base part upper position O as the center, radial folds P are provided, and the air bag 17 is stored being folded inward in a roll wound onto the vehicle interior side so as to get close to the base part 25 side from the upper edge 34 side.