A side air bag apparatus installed within a seat has an air bag unit containing an inflator and a folded air bag in an air bag unit housing which is connected to a firing control unit by way of a harness partly extending from the air bag unit within and along a lower transverse section of the seat from one side of the seat close to a door to the other side of the seat and partly extending under the seat to the firing control unit via a coupler secured to the underside of the seat.