The invention has as its object to provide an air bag (11) for a side air bag device capable of reducing the size and weight of the device (M1) while retaining a tension line (L - L) when expanded. The air bag (11) is mounted on the peripheral edge of a door opening (W) at the inside of a compartment, and is formed into a generally rectangular bag shape for covering the door opening when expanded, while being equipped with a gas inlet (19) for introducing an expanding gas. The air bag is provided with joints (22) jointing an inner side wall (12) and an outer side wall (13) partially so as to form a tension line L - L when expanded. The individual joints (22) are juxtaposed in plurality along the tension line so as to cross the tension line, and are arranged generally at vertically middle portions so as to form one expansion chamber (18) having upper and lower spaces (18a and 18b) for the expanding gas to flow therethrough.
