The side impact airbag includes an airbag that is folded up from a state where an occupant-side wall and a vehicle-side wall having generally identical contours are lapped and flattened, through a vertical contracting step. The airbag includes at least either at the top or bottom a tucked portion that is formed in the vertical contracting step by tucking i.e. invaginating part of each of the occupant-side wall and the vehicle-sidewall inside the airbag. The tucked portion includes a main portion that is stored inside the airbag and a projecting region that extends vertically from the main portion in an exposed manner. The projecting region is folded back toward the vehicle-side wall so as to be located outside the main portion.