An air bag against side collision, wherein two faces of mutually opposed peripheral wall portions having a substantially rectangular shape are connected by tethers in a strip or band-like shape whereby a distance of separation between the side wall portions in expanding the air bag is restricted. The expanded shape of the air bag is a substantially rectangular plate having four edges. An inner location near a rear edge accommodates an inflator supplying gas for expansion. Two side edges extend substantially parallel with each other from opposite ends of the rear edge and a front edge In a folded state, the front edge side of the air bag is cactus-folded such that the front edge side enters the inner side of the rear edge side of the air bag and approaches the rear edge. Tethers are connected respectively to the peripheral wall portions with a width dimension parallel with folds where the front edge side of the air bag is drawn in.