A side airbag apparatus (100, 100A-100G) installed in a vehicle, comprising: a bag shaped cushion (114, 114A) which is housed in a seat back (104, 112) of a vehicle seat and expanded and deployed on the side of the vehicle seat utilizing gas supplied from an inflator; and a holding part (128, 138A, 138B, 150, 158, 176, 178, 180) for retaining the shape in the housed state of the cushion (114, 114A); wherein the cushion (114, 114A) is wound or folded in a long shape in the vertical direction of the vehicle and housed in the seat back (104, 112) in the shape with the upper part or lower part thereof further folded back to the anterior of the vehicle, and wherein the holding part (128, 138A, 138B, 150, 158, 176, 178, 180) retains the upper part or lower part of the cushion (126, 174) which is folded back to the anterior of the vehicle, wherein the holding part (128, 138A, 138B, 150, 158, 176, 178, 180) is a flexible cloth material, characterized in that the holding part (128, 138A, 138B, 150, 158, 176, 178, 180) includes: a main body part (182A, 182B) which is long shaped in the vertical direction and bag shaped when the cloth material is sewn, in addition to having a non-sewn part (146, 194, 194A, 194B) which is not sewn, with the cushion (126, 174) capable of being placed from the outside to the inside; a first flap part (184, 184A) which extends from the non-sewn part (146, 194, 194A, 194B) of the main body part (182A, 182B) toward the front of the vehicle and covers a region not overlapping the folded back upper part or lower part of the cushion (126,174) when folded back to the posterior of the vehicle; and a second flap part (186, 186A) which extends from the non-sewn part (146, 194, 194A, 194B) of the main body part (182A, 182B) toward the front of the vehicle, is disposed upwards in the vehicle or downwards in the vehicle compared with the first flap part (184, 184A), has long dimensions in the anteroposterior direction of the vehicle, and covers a region not overlapping the upper part or lower part of the cushion (126, 174) folded back to the anterior of the vehicle when folded back to the posterior of the vehicle. a bag shaped cushion (114, 114A) which is housed in a seat back (104, 112) of a vehicle seat and expanded and deployed on the side of the vehicle seat utilizing gas supplied from an inflator; and a holding part (128, 138A, 138B, 150, 158, 176, 178, 180) for retaining the shape in the housed state of the cushion (114, 114A); wherein the cushion (114, 114A) is wound or folded in a long shape in the vertical direction of the vehicle and housed in the seat back (104, 112) in the shape with the upper part or lower part thereof further folded back to the anterior of the vehicle, and a main body part (182A, 182B) which is long shaped in the vertical direction and bag shaped when the cloth material is sewn, in addition to having a non-sewn part (146, 194, 194A, 194B) which is not sewn, with the cushion (126, 174) capable of being placed from the outside to the inside; a first flap part (184, 184A) which extends from the non-sewn part (146, 194, 194A, 194B) of the main body part (182A, 182B) toward the front of the vehicle and covers a region not overlapping the folded back upper part or lower part of the cushion (126,174) when folded back to the posterior of the vehicle; and a second flap part (186, 186A) which extends from the non-sewn part (146, 194, 194A, 194B) of the main body part (182A, 182B) toward the front of the vehicle, is disposed upwards in the vehicle or downwards in the vehicle compared with the first flap part (184, 184A), has long dimensions in the anteroposterior direction of the vehicle, and covers a region not overlapping the upper part or lower part of the cushion (126, 174) folded back to the anterior of the vehicle when folded back to the posterior of the vehicle.