A side airbag apparatus is provided with an airbag which is stored within a vehicle seat. A rear portion of the airbag is fixed to the vehicle seat. Further, the side airbag apparatus is provided with an inflator which is arranged within a rear portion of the airbag and is fixed to the vehicle seat. The side airbag apparatus jets out gas from the inflator in correspondence to an impact applied to a vehicle from a side direction, and inflates and deploys the airbag between a body side portion and the vehicle seat. Further, a vent hole for discharging the gas within the airbag is provided between a gas outlet of the inflator and at least one of an upper end and a lower end of a rear portion of the airbag. Further, the airbag is folded in such a manner as to start and finish an inflation of the airbag finally in the vicinity of the vent hole between the vent hole and the gas outlet.