A side airbag apparatus provided with an airbag cushion installed in an armrest side portion of a seat frame of a vehicle, may include an airbag module having the airbag cushion therein; and a strap fastened at a first end portion thereof to a portion of the airbag cushion and fastened at a second end portion thereof to the seat frame or a bolster support wire frame, the strap surrounding a front portion of the seat frame.